The Importance of Water Hardness for Our Fish
- Macauley Sykes
- Oct 5
- 26 min read
Updated: Oct 6
Ask any aquarist what matters most in their water, and you’ll likely hear the same list: pH, ammonia, nitrite, and nitrate. These are the familiar parameters for aquarium water, the ones we test, track, and talk about endlessly. But lurking quietly in the background is another parameter that shapes everything in the aquarium, a factor so fundamental that it affects every breath, heartbeat, and cell function your fish perform.
That hidden force is water hardness. Many aquarists glance at it once on a test strip and move on, assuming it’s a technical detail best left to breeders or scientists. In truth, hardness is far from a background number.
Water hardness controls osmoregulation, the process by which fish maintain the delicate balance of water and salts across their gills and skin.
It provides the calcium and magnesium that underpin muscle movement and bone growth. It buffers pH, preventing sudden crashes that can devastate an entire tank. And in breeding, the correct mineral balance can mean the difference between clear, fertilised eggs and a batch that never develops at all.
It even determines whether your filter bacteria can function effectively.
So why does something so important so often get ignored?
Partly because hardness is poorly understood.
Many beginners and even some experienced hobbyists focus predominantly on ammonia, nitrite, nitrate, and pH, viewing these as the only indicators of tank health. General Hardness (GH) and Carbonate Hardness (KH) are often lumped together, mislabelled, or confused with pH itself. Online discussions are also full of vague references to “soft” and “hard” water, without context, without numbers, and without the science that gives those terms meaning.
Additionally, somewhere along the line, a myth took hold that today’s farm-bred fish no longer care what’s in their water, that evolution has somehow caught up with the hobby. But it hasn’t.
Hardness still matters, every bit as much today as it ever did.
It is the unseen chemistry that governs life beneath the surface, from the mineral-depleted blackwaters of the Amazon to the limestone shores of Lake Malawi. When you learn to understand and control it, everything in the aquarium begins to make more sense. Stability improves, colours intensify, behaviour becomes more natural, and breeding success often follows.
Understanding hardness isn’t just a technical step forward. It is an 'aha moment' for any aquarist. It turns water from something we merely fill our tanks with into something we truly work with.
What is Water Hardness?
When we talk about water hardness, we’re really talking about the minerals dissolved in the water.
In aquariums, hardness isn’t just a chemical curiosity or a number on a test kit. It directly shapes the environment that fish live, breathe, and grow in. Hardness affects how fish regulate salts in their bodies, how their skeletons form, and how stable the water’s pH remains from day to day.
There are two main types of hardness that aquarists need to understand:
General Hardness (GH)
General Hardness (sometimes called permanent hardness) measures the total concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions dissolved in water. These are the minerals that play a direct and vital role in the physiology of fish and other aquatic life. Just as animals rely on calcium for strong bones and magnesium for healthy muscles, fish depend on these minerals to support essential biological functions too. GH influences osmoregulation (the process by which fish balance salts and fluids in their bodies) as well as bone and scale formation, muscle contraction, enzyme activity, and nerve function. Hardness also plays a subtle but crucial role in reproduction. Many softwater fish lay delicate eggs that are adapted to extremely low mineral content, while hardwater species rely on calcium-rich conditions to ensure proper egg hardening and sperm motility. Even minor shifts in GH can affect spawning success and fry development.
Aquarium plants and invertebrates also draw from these minerals.
GH helps maintain healthy snail shells, shrimp exoskeletons, and the structural integrity of plants.
Carbonate Hardness (KH)
Carbonate Hardness (sometimes called alkalinity) measures the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions dissolved in water.
KH acts as a buffer, preventing sudden pH drops, keeping the aquarium environment stable and safe for life.
Think of KH as a shield that protects pH. Carbonates and bicarbonates act like cushions, neutralising acids as they form from respiration, waste breakdown, and biological filtration. With this protective buffer in place, the water resists sudden shifts, keeping pH steady and predictable. Without it, every small input of acid pushes pH lower, sometimes in dramatic “crashes” that can stress or kill fish.
KH also plays a crucial role in biological filtration. The nitrifying bacteria that convert toxic ammonia into less harmful nitrate consume carbonate ions as part of their metabolism. When KH becomes depleted, these bacteria slow down or even stall, weakening the biofilter and allowing ammonia and nitrite to rise.
Where does Water Hardness come from?
Every drop of freshwater has a story, and that story begins with the rain.
When rain forms, it starts almost perfectly pure. It carries nothing but trace gases from the atmosphere: oxygen, nitrogen, and a small amount of carbon dioxide. That carbon dioxide dissolves into the droplets, forming a weak carbonic acid that makes rain slightly acidic, usually around pH 5.5–6.0. At this point, the water is extremely soft, holding virtually no dissolved minerals. It is clean, empty, and waiting to be shaped by the world below.
As it falls and begins its journey through the landscape, rainwater becomes a natural solvent. It flows across rocks, seeps through soil, and percolates through cracks and sediments, dissolving minerals along the way. The geology of the land decides everything from that moment on.
When rainwater runs over or through limestone, chalk, or dolomite, it begins to absorb calcium and magnesium ions from the rock. This process can enrich the water with dissolved minerals, making it harder and more alkaline than it was before. How hard or alkaline the water becomes depends on many factors, such as the local geology, the volume and speed of water flow, and how long that water remains in contact with mineral-bearing rock.
In East Africa, the vast Rift Lakes (specifically Malawi and Tanganyika) sit within basins lined with carbonate-rich rock. Over thousands of years, water has dissolved minerals from these ancient formations, creating lakes that are crystal clear, highly mineralised, and remarkably stable. Conductivity often exceeds 600 µS/cm, with GH values above 10°dH and pH between 7.8 and 9.0.
These conditions have shaped one of the most diverse fish radiations on Earth: the Rift Lake cichlids. Their colours, breeding behaviours, and even jaw structures have evolved to match this mineral-rich, alkaline environment. Similarly, Central American limestone rivers provide the perfect home for livebearers such as guppies, mollies, and swordtails, species that depend on calcium for skeletal growth and fry development.
The Mbu Puffer (Tetraodon mbu) is also native to Lake Tanganyika and the drainage rivers that flow into and out of it, where the water remains hard and alkaline. These conditions have shaped its physiology and behaviour, creating a species adapted to high alkalinity, abundant buffering capacity, and unwavering pH stability. It is a true product of Tanganyika’s remarkable geology.
Where the bedrock is granite, sandstone, or quartz, the story changes entirely. These rocks are far less soluble than limestone or dolomite and contribute almost no minerals to the water that passes over them. As a result, the rivers and streams that drain such landscapes remain soft and often slightly acidic, with minimal buffering capacity. Their chemistry is governed not by dissolved minerals, but by organic matter leaching from forests and soils.
This is where some of the world’s most distinctive freshwater systems begin, the blackwaters. As rainwater seeps through layers of fallen leaves, roots, and ancient peat, it becomes enriched with organic acids such as humic and fulvic acids. These acids stain the water a deep amber or tea colour and further reduce its mineral content. Rather than collecting hardness, the water becomes even softer, its GH and KH often close to zero.
The Rio Negro in the Amazon is perhaps the most famous blackwater system. Its inky colour comes from rainwater filtered through vast tracts of rainforest and peat-rich soils. Here, the conductivity is almost negligible, and the water holds so little mineral content that it can barely register on a hardness test. Across the world in Borneo, Sumatra, and the Congo Basin in Africa, the story repeats in tropical peat swamps shaded by dense forest canopy. These waters, equally soft and acidic, are home to labyrinth fish such as gouramis and bettas, as well as rasboras and loaches, species perfectly adapted to thrive in what would be too extreme for most fish.
The Red-Tailed Puffer (Carinotetraodon irrubesco) is native to the peat swamps and forest streams of southern Borneo and Sumatra, where the water runs dark with tannins and is almost devoid of minerals. In these quiet blackwater systems, pH often falls below 5.0, and hardness is virtually unmeasurable. The species has evolved to thrive in such soft, acidic conditions, living in stability not through buffering, but through the near absence of dissolved salts.
Over millions of years, differences in water chemistry have guided evolution itself. The Mbu and the Red-Tailed Puffer stand at opposite ends of the mineral spectrum. In Tanganyika, geological wealth has given rise to a giant; in the peat swamps of Borneo, scarcity has produced a miniature. Hardness and buffering shape more than numbers on a test kit. They define the scale, physiology, and even the possibilities of life within the water.
Fish did not simply adapt to water; they were sculpted by it. Their physiology, colours, breeding behaviour, and social structures all reflect the chemistry of the environments that shaped them.
From the rain that falls to the rivers that flow, every journey of water tells part of that story. The world’s fish are living reflections of it, shaped and defined by the character of the waters that gave them life.
Water Hardness in the Modern Aquarium Trade
Every fish carries the memory of its natural habitat. Even those bred in captivity still bear the imprint of the waters their ancestors came from. The minerals, acidity, and balance of their native rivers shaped their biology over countless generations, defining how they breathe, grow, and reproduce.
In the wild, water hardness isn’t just a number; it is part of the environment’s identity.
Yet in the modern aquarium trade, it is often treated as an afterthought, easily overshadowed by pH, temperature, or filtration.
Most of the freshwater fish seen in aquarium stores today are no longer wild-caught but bred on farms across Asia, Europe, and the Americas. Aquaculture now supplies over ninety per cent of the freshwater species in the ornamental trade, and these farms seldom rely on their natural water supply. Instead, they use advanced systems to shape and stabilise water chemistry with remarkable precision, adjusting hardness, temperature, and mineral balance to suit each species and stimulate breeding.
Within these environments, every parameter is monitored and refined. The result is a stable, repeatable setting where fish can be bred predictably and efficiently. Breeders are no longer constrained by the hardness or softness of their local water, nor by the rhythm of seasonal rains or droughts. Controlled systems allow reproduction year-round, wherever in the world the facility may be.
This precision lies at the heart of modern aquaculture. It allows breeding centres to produce species from every corner of the world, all under one roof. The success of today’s ornamental fish trade depends not on the natural qualities of a region’s water, but on the ability to recreate and override them entirely.
In breeding facilities, water chemistry is something to be engineered and perfected.
In retail, however, it is often forgotten.
Most aquarium stores operate large, centralised systems built for efficiency rather than authenticity. Dozens, sometimes hundreds, of tanks are connected to the same filtration line, circulating a shared body of conditioned tap water that reflects the chemistry of the local supply. Whether that water is naturally hard or soft depends entirely on the region, not on the needs of the fish within it. Because every tank is linked, all species are exposed to the same conditions. For retailers, it is far more practical to maintain one consistent set of parameters than to adjust the water for each species individually.
In the retail world, water is a medium of storage, not an environment of belonging.
From a business standpoint, this approach makes sense. Maintaining separate systems with water tailored to different species or regions would be impossibly expensive and time-consuming. This is why the rows of tanks in most shops are dominated by species that can survive outside their ideal parameters for a time without showing obvious signs of distress. Livebearers, barbs, danios, and common tetras form the backbone of the trade precisely because they endure what more sensitive fish cannot.
Delicate species, by contrast, fare poorly in such generalised systems. Those that depend on narrow chemical stability often decline quickly, and with higher mortality rates come reduced profit margins. As a result, these fish are rarely seen in quantity. Many are imported only in small numbers, while others disappear from mainstream retail altogether, quietly replaced by species that can better withstand the compromises of commercial holding.
Captive breeding has introduced some tolerance for parameters that fall outside the natural range for certain fish, but this resilience comes through selective survival rather than a rewriting of biology. These fish still carry the same physiology as their ancestors, finely tuned for the habitats in which they evolved. Studies on guppies, bettas, and zebrafish - staples of the aquarium trade - show that when hardness strays too far from the optimum range, fertility declines, fry deformities increase, and lifespan shortens.
This is one reason many fish live only a fraction of their potential lifespan in captivity.
Neon tetras, for example, are capable of living for ten years in excellent conditions, yet most survive only two or three. The decline is gradual and easily missed: colours fade, behaviour dulls, and vitality wanes, often mistaken for normal ageing rather than the quiet toll of chronic environmental stress.
Yet understanding this doesn’t make fishkeeping more complicated; it makes it more meaningful. Hardness isn’t a number to control, but a foundation to respect.
Matching Fish to Your Tap Water
Water isn’t just the space your fish live in. It is part of what they are. Every heartbeat, every exchange of oxygen, every electrical signal in a muscle fibre depends on the minerals dissolved within it.
Water doesn’t just support life. It defines it.
And because of that, understanding your water is one of the most valuable skills an aquarist can learn.
Before you even add your first fish, the chemistry of your source water has already decided which species will flourish effortlessly and which will struggle against the odds. For most aquarists, the simplest path to success isn’t to reinvent their water, but to work with it. By testing GH and KH, you reveal its natural character, whether it is soft and slightly acidic or hard and alkaline. Once you know where it sits on the scale, choosing fish that truly suit it becomes far easier. It allows you to select species that were born to thrive in water just like yours.
The goal isn’t to label your water as soft or hard, but to understand where it falls along the hardness spectrum, and to choose fish that evolved within that same window of chemistry. That is where harmony begins.
If your water is naturally soft, it may provide a comfortable home for species that come from mineral-poor habitats, such as tetras, rasboras, and gouramis that thrive in gentle, slightly acidic conditions. These species often display richer colouration, calmer behaviour, better spawning results, and longer lifespans when kept within their natural range.
Harder, more alkaline water tells a different story. It supports fish that have evolved to depend on dissolved minerals for health and growth, including livebearers, many rainbowfish, and certain cichlids. Each species has its own place within the range, and even small differences in hardness can influence long-term health.
Understanding your water is the foundation of every successful aquarium. Once you know its chemistry, you stop guessing and start building with purpose. Matching fish to the conditions they were born for doesn’t limit your creativity; it enhances it. It is what turns a tank from a collection of species into a living, breathing ecosystem where everything feels balanced and at ease.
And if your dream fish comes from water unlike what comes from your tap, that knowledge doesn’t close a door. It opens one, an invitation to learn how to shape your water with intention and precision, and to recreate, as faithfully as possible, the environments that shaped those fish in the wild.
Creating Water to Match Your Fish
Most aquarists build their success on a simple principle: work with the water you have. For many species, that is all they will ever need. But some fish come from environments so distinct that their lives are shaped by water very different from what any of us have flowing from our taps.
When that is the case, the aquarist’s role becomes more creative. Instead of asking the fish to adapt to our conditions, we learn to shape the water to match theirs. It isn’t about control or complexity. It is about care, and about recreating an environment that allows fish to live as nature intended, showing the same colours, behaviours, and quiet ease they display in the wild.
To many, this begins with blackwater.
These are the rainforest streams of Borneo, the peat swamps of Malaysia, and the forest pools of the Amazon. Their waters are stained the colour of tea by tannins and humic acids, with conductivity so low that electronic meters often read zero. Beneath the canopy, the substrate is soft, shaded, and covered with leaf litter. Minerals are scarce, and pH can fall below 4. For the aquarium, such water cannot be drawn from a tap. It must be built.
Starting with reverse osmosis water provides a blank slate. A small measure of calcium and magnesium is then added to establish the faint trace of hardness that even the softest ecosystems contain. From there, the aquarist introduces botanicals, alder cones, or peat to release organic acids that gently lower pH and bind trace metals. Over time, the water darkens to a warm amber hue, light filters through it in soft ribbons, and the fish respond with relaxed movement and subtle, natural colour. What was once chemistry becomes atmosphere, a living reflection of their wild world.
At the other extreme lie the great rift lakes of Africa. Lake Malawi and Lake Tanganyika are vast inland seas, rich in dissolved minerals and held in perfect chemical balance by their limestone basins. Their pH is often above 8, their hardness is among the highest on Earth, and their water is so clear that light penetrates to extraordinary depths.
In captivity, this environment too must be crafted. Aquarists raise hardness and buffering by adding aragonite, crushed coral, or specialised mineral salts. The result is a stable, alkaline system that supports the intense colours, bold behaviours, and prolific breeding that define these fish in nature.
These two examples sit at opposite ends of the aquatic spectrum, yet they share the same philosophy: respect the biology of the fish through maintaining the water parameters they evolved in.
Every aquarist who learns to shape water - whether soft and acidic or hard and alkaline - steps into a tradition that stretches back to the earliest naturalists. We become, in a sense, caretakers of chemistry, guiding invisible forces so that life can unfold as it should. Every drop we mix, every test we take, every adjustment we make brings us closer to the rivers, forests, and lakes from which our fish came. That is the true art of fishkeeping.
General Hardness (GH)
Every species of fish has evolved within a particular mineral landscape.
The rivers, lakes, and floodplains that shaped them differ not only in colour and flow but also in the hardness of their water. Some come from rain-fed forest streams where minerals are scarce, while others inhabit ancient rift lakes where every drop is rich with dissolved calcium and magnesium. These minerals form the invisible framework of aquatic life - shaping how fish breathe, grow, and reproduce - and their importance in the aquarium is no different from the wild.
General hardness (GH) describes the concentration of essential calcium (Ca²⁺) and magnesium (Mg²⁺) ions dissolved in water.
In fish, calcium supports bone, scale formation, while magnesium drives the enzymes that power movement, feeding, and growth. Both are essential to osmoregulation, the process by which fish maintain equilibrium between their bodies and their surroundings.
Plants depend on the same minerals. Magnesium forms the heart of every chlorophyll molecule, capturing light for photosynthesis, while calcium fortifies new cell walls and prevents distortion in growing tissue. Without these elements, fish tire easily, plants weaken, and the entire ecosystem loses its delicate balance.
When GH sits comfortably within the range their biology expects, fish breathe more easily, display richer colouration, and behave with quiet confidence. Plants grow stronger and more upright, their leaves more vibrant and resilient.
Even the unseen microbial life within the filter and substrate depends on calcium and magnesium. These ions strengthen cell walls, stabilise enzymes, and help bind biofilms to their surfaces, giving the bacterial colonies that purify the water a firm chemical foundation.
Together, they form the invisible structure that supports all life within the aquarium.
Increasing General Hardness
Every aquarium, regardless of style or setting, requires some degree of general hardness. Without it, the biological engine that drives the system, from microbial life to fish metabolism, cannot function as it should. When water contains too little calcium and magnesium, it lacks the mineral foundation on which every form of aquatic life depends.
You may find that the general hardness of your source water is lower than the needs of the fish and plants you keep. In such cases, these minerals must be added to secure the long-term health of the inhabitants. Yet the amount of hardness needed is not universal. It depends entirely on the environment your fish evolved in and where they fall along the spectrum of hardness.
Setting the Right Target
Before raising GH, it is essential to understand the mineral range your fish require.
Each species has evolved to thrive within a particular balance of calcium and magnesium, shaped by the geology of its native waters.
Soft-water species such as tetras, rasboras, gouramis, and most dwarf cichlids flourish between roughly 2 and 6 °dGH.
Moderate-water species like barbs, loaches, and many community fish prefer around 5 to 10 °dGH.
Hard-water fish, including livebearers, rainbowfish, Rift Lake cichlids, and many brackish species, do best between about 10 and 20 °dGH.
These are not rigid rules but biological guidelines. The goal is to bring GH comfortably within the range that mirrors each species’ natural environment, allowing them to live and function as nature intended.
Maintaining hardness within that window supports long-term health and breeding success, while pushing too far beyond it can lead to subtle stress, reduced fertility, or a shortened lifespan, even if the fish appear healthy at first glance.
The Simplest and Most Accurate Method of Increasing/Adding GH
In the aquarium, general hardness is raised by adding carefully measured amounts of calcium and magnesium salts.
These minerals form the foundation of biological health and can be adjusted with precision and safety using two simple compounds:
Calcium sulphate dihydrate (CaSO₄·2H₂O) - commonly known as gypsum
Magnesium sulphate heptahydrate (MgSO₄·7H₂O) - widely available as Epsom salt
Both are safe, inexpensive, and ideal for the fine control of GH.
Together, they replace the calcium and magnesium ions found in natural waters without increasing carbonate hardness (KH) or altering pH.
Dosing
In most natural waters, calcium outweighs magnesium by roughly three to one. Maintaining this natural balance ensures a mineral profile that supports both fish and plants, promoting strong bone and tissue formation in animals and steady, resilient growth in aquatic vegetation.
To raise 1 °dGH in 100 litres, use 2.5 g of gypsum and 0.8 g of Epsom salt, maintaining the natural 3:1 ratio of calcium to magnesium.
These amounts raise GH by roughly 17.9 ppm (1 °dGH).
Quick reference per 1 °dGH
50 L: 1.25 g gypsum + 0.42 g Epsom
100 L: 2.50 g gypsum + 0.83 g Epsom
200 L: 5.00 g gypsum + 1.66 g Epsom
400L: 10.00 g gypsum + 3.32 Epsom
It is best to premix all new tank water before it goes into the aquarium. However, if you are dosing the aquarium directly to make small adjustments, first dissolve the minerals completely in a separate container of water. Add the solution gradually into a high-flow area to prevent localised concentration and to allow the minerals to disperse evenly.
Raise hardness by no more than 1–2 °dGH per 24 hours, and re-test between additions. This gives fish and microorganisms time to acclimate safely.
Ongoing upkeep
Once the aquarium has reached its target GH, the goal is to maintain that level rather than repeatedly adjust it.
Minerals are slowly removed by plant growth, microbial use, and water changes.
A mild downward drift in GH is normal. Test regularly and add small top-ups to replacement water to keep GH within your target range.
Consistency and patience maintain balance. Over time, this becomes a calm routine that restores strength and structure to the water without disturbing the life within it.
Safety notes
Always choose food-grade or laboratory-grade salts.
Avoid agricultural or construction materials, which can contain impurities such as heavy metals or binding agents. Food-grade Epsom salt is readily available from pharmacies, while gypsum can be sourced from reputable aquarium suppliers, hydroponic outlets, or food-additive retailers.
Never tip dry salts straight into a stocked aquarium because the sudden localised concentration can shock fish and disturb the biological balance.
Commercial Mineral Blends
For convenience, several commercial mineral formulations are available that contain pre-balanced mixtures of calcium, magnesium, and trace elements. These products are designed to simplify the process of remineralising reverse-osmosis or very soft water. They can be highly effective, provided they are used with accuracy and tested regularly.
The main advantage of commercial blends is consistency.
Each dose adds a predictable level of hardness and trace minerals without the need to weigh separate salts. The limitation is control: you cannot independently adjust calcium and magnesium ratios, and many formulations contain additional elements such as potassium or iron that may not be required in every aquarium.
For keepers who prefer full precision, using pure salts remains the most flexible approach.
For those seeking simplicity and reliability, a well-chosen remineralising blend offers an easy, dependable alternative. In either case, the goal is the same: to provide the mineral foundation that supports all life within the aquarium.
Carbonate Hardness (KH)
Carbonate hardness (KH) is what keeps pH stable. It acts as a buffer, absorbing both acids and bases and preventing the water from becoming too acidic or too alkaline. Every aquarium depends on this quiet balance. A steady KH protects fish and microorganisms from the daily shifts in chemistry that naturally occur as life within the tank breathes, feeds, and grows.
When KH falls too low, pH becomes unstable and begins to drift.
In aquariums rich in wood, peat, or leaf litter, organic acids are released as these materials decompose, consuming carbonate and pushing pH downward.
In contrast, tanks with strong aeration or dense plant growth can swing the other way. As carbon dioxide is stripped from the water or consumed during photosynthesis, the loss of carbonic acid allows the pH to rise. Without buffering, both directions are possible, and even gentle swings can place stress on fish and reduce the efficiency of biological filtration.
For most aquariums, there is no need to artificially raise KH.
A moderate level of between 3–6 °dKH provides ample buffering to prevent meaningful pH swings under normal conditions. As long as regular water changes are carried out to replenish carbonate and the system is not overloaded with organic material, that level of carbonate hardness will keep the water stable and predictable.
When 3–6 °dKH is sufficient:
Moderate biological load: A typical community aquarium with a reasonable number of fish and a standard maintenance routine produces acids at a manageable rate. The KH buffer in this range is strong enough to neutralise these acids and keep the pH stable.
Consistent water changes: Regular water changes are key to this process. If the tap water used for changes has a reasonable KH (in this range or higher), it will naturally keep the tank's buffering topped up through water changes. They not only remove waste but also replenish the consumed buffers, helping to prevent the gradual decline of KH and staving off 'Old Tank Syndrome'.
When to Adjust KH
For most freshwater aquaria, KH rarely needs adjustment.
A moderate level is enough to maintain a stable pH under normal conditions. As long as water changes are routine and the system isn’t overloaded with decaying material or excessive bioload, that buffer is more than sufficient.
Situations where KH adjustment can be useful include:
Heavily Planted, CO₂-Injected Systems
In aquaria where carbon dioxide is added, KH determines how much the pH fluctuates as CO₂ levels change between day and night. A slightly higher KH (typically 4–6 °dKH) helps prevent large pH swings that could otherwise stress fish and invertebrates.
Environments with Strong Acid Production
Tanks that contain large amounts of leaf litter, peat, or other organic material constantly release humic and tannic acids. In such cases, KH can slowly be consumed. If it falls close to zero, the buffering system collapses, and the pH may crash. A small, controlled increase in KH stabilises conditions without compromising the soft, acidic nature of the water.
Species from Alkaline or Hard-Water Habitats
Some species genuinely depend on high carbonate levels because that reflects their native chemistry. This includes Rift Lake cichlids, Central American livebearers, and other fish adapted to mineral-rich environments. For these aquaria, a higher KH forms part of accurately recreating their native chemistry, alongside elevated GH and pH.
Outside of these special cases, natural equilibrium is best. Most community and soft-water setups remain perfectly stable with a KH of 3–4 °dKH, provided that water changes are regular and organic load is managed.
Does KH Matter to the Fish Themselves?
Only indirectly in freshwater systems. For most freshwater species, carbonate hardness (KH) is not a parameter that directly influences their internal physiology. Freshwater fish live in an environment that is much less saline than their own body fluids, so they constantly absorb water and lose ions through diffusion. Their osmoregulatory processes revolve around conserving salts such as sodium, potassium, calcium, and magnesium, not bicarbonate.
As long as pH is stable and mineral levels (GH) are appropriate, the precise KH value makes little biological difference to them. Its main function is chemical, not physiological.
By contrast, marine and brackish fish inhabit environments rich in carbonate and bicarbonate ions. For these species, bicarbonate contributes to acid–base balance and osmoregulation within the gills and bloodstream.
In such systems, maintaining carbonate alkalinity is directly tied to the animal’s health, but that is a property of the marine environment itself, not of carbonate hardness as we measure it in freshwater aquaria.
Old Tank Syndrome
Over time, the chemistry of every aquarium changes. Minerals are slowly consumed, organic waste accumulates, and the buffering that keeps water stable begins to fade.
Carbonate hardness (KH) is gradually used up as acids are produced through respiration, biological filtration, and the natural decay of organic matter. When this buffer is exhausted, the pH starts to fall, and the environment becomes increasingly acidic.
As the pH drops, the biological filter begins to struggle. The bacteria responsible for converting ammonia to nitrate depend on an alkaline environment to function. Their activity slows markedly below pH 6.5, becomes weak below pH 6, and almost stops altogether near pH 5.5. It is not the absence of KH itself that halts filtration, but the acidic conditions that develop once the buffer is gone.
During this time, ammonia excreted by fish is converted into ammonium (NH₄⁺).
Ammonium is far less toxic, so it often goes unnoticed, but it remains in the water, waiting.
When a large volume of fresh, mineral-rich water is added too quickly, the pH rises, and the stored ammonium is converted back into the much more toxic ammonia (NH₃) within minutes, overwhelming the weakened biological filtration and exposing the fish to toxins in addition to subjecting them to a violent pH climb.
This process is sometimes referred to as Old Tank Syndrome, and it is simply what happens when hardness and buffering are allowed to decline unchecked. When fish die suddenly after a long-delayed or overly large water change in an overstocked or poorly maintained aquarium, it is usually this chemistry at work. Months of acid accumulation and depleted buffering leave the system fragile; the sudden reintroduction of alkaline water transforms harmless ammonium into lethal ammonia within minutes, exposing the fish to toxins in addition to subjecting them to a violent increase in pH.
The solution is not complicated; it is consistency.
Regular water changes replenish lost minerals, restore carbonate buffering, and keep the biological filter active. Testing KH and pH as part of your normal maintenance routine will reveal any gradual drift long before it becomes a problem.
A well-maintained tank should never develop Old Tank Syndrome.
By renewing a portion of the water each week, you maintain the same quiet balance found in nature: stable chemistry, steady filtration, and a system that thrives through gentle, ongoing care.
Practical Method for Raising KH
The most reliable way to increase carbonate hardness is with potassium bicarbonate (KHCO₃).
When dissolved in water, it releases bicarbonate ions that raise KH in a controlled and predictable manner, strengthening the buffer without adding unwanted elements such as sodium or calcium.
Unlike sodium bicarbonate (baking soda), which can lead to sodium accumulation over time, potassium bicarbonate contributes an essential plant nutrient. It supports healthy leaf growth in planted aquaria while gently stabilising pH and biological function.
As a general guide, 3 grams of potassium bicarbonate per 100 litres of water will raise KH by approximately 1 °dKH (17.9 ppm).
Always dissolve the powder completely in a jug of water, then pour the solution into your mixing container or water butt. Test the KH and pH before adding the water to the aquarium, ensuring it closely matches the conditions already in the tank.
For routine maintenance, buffer your replacement water during each water change so the KH remains steady. If you need to increase KH directly in the aquarium, do so very slowly - adding the fully dissolved solution into a high-flow area, and raising hardness by no more than 1–2 °dKH per day. Re-test after several hours to confirm stability before adding more.
Avoiding Rapid pH Rises
Adding bicarbonate raises KH but also neutralises acid, which in turn raises pH. If done too quickly, this can expose fish and microorganisms to an abrupt increase, which can cause shock.
To prevent this:
Pre-buffer new water in a separate container. Adjust KH and test pH before adding it to the aquarium. The pH of the new water should be within 0.2–0.3 units of the tank’s current value.
Add in stages. Split your total dose into smaller portions given over several hours or even days. This allows natural CO₂ exchange to stabilise the pH between additions.
Encourage gentle aeration. Moderate surface movement helps excess CO₂ escape slowly, preventing sudden chemical shifts.
Re-test before each addition. pH may continue to rise after dosing, especially in low-buffered water. Wait for readings to stabilise before adding more.
Never mix bicarbonate powder directly into the tank. Localised alkalinity spikes can form around undissolved grains and irritate gills or damage biofilms.
Handled in this way, increasing KH becomes a controlled, predictable process. Fish experience only gentle, gradual changes, the kind of stability they’re adapted to in nature.
Other Methods and Their Drawbacks
Sodium bicarbonate (baking soda) also raises KH but introduces sodium, which accumulates over time and can stress soft-water species. It is best reserved for short-term corrections or emergency buffering.
Calcium carbonate, aragonite, crushed coral, and limestone dissolve slowly, releasing both carbonates and hardness minerals.
However, their behaviour can be unpredictable. The rate at which they dissolve depends on pH and CO₂ levels. When the water is soft or acidic, they dissolve rapidly; when the pH is high, they dissolve very little.
How to lower GH and KH
Across the world, tap water reflects the land it travels through. In some regions, it passes through layers of chalk, limestone, or dolomite, gathering calcium and magnesium until it emerges rich and alkaline. This mineral-heavy water is ideal for livebearers, rainbowfish, and Rift Lake cichlids, yet for species from softer, rain-fed habitats, it can feel dense and unnatural.
Softening water brings it closer to the chemistry of those quieter places: blackwater creeks, forest pools, and slow tributaries where minerals are scarce and the water feels light. It is not about stripping water bare, but about removing the excess so that life can move and breathe as it was meant to.
The Most Reliable Method of Lowering GH and KH
The simplest and most predictable way to reduce hardness is through dilution with pure water. Reverse osmosis (RO), deionised, or distilled water contains almost no dissolved minerals, making it the most effective base for softening tap water. When mixed, the two waters form a blend with proportionally lower hardness.
The relationship between dilution and hardness is linear.
If your tap water measures 10 °dGH and 6 °dKH (for example), blending it 50:50 with pure RO water will produce roughly 5 °dGH and 3 °dKH.
A 75:25 mix (three parts RO to one part tap) would lower it further, to around 2–3 °dGH and 1–2 °dKH.
When you dilute water, you are not only reducing GH and KH, but also everything else dissolved within it. Buffers, trace elements, nutrients, and impurities are all reduced equally.
Each water supply behaves differently, so the most effective approach is to test and refine your own ratio. Start by measuring the GH and KH of your tap water, then blend it with pure water in different proportions until you achieve the desired range. Once the correct mix is found, record the ratio and repeat it for every future water change. This ensures consistency and allows you to maintain the same chemistry indefinitely.
Softening with Natural Materials
Some aquarists also soften water by introducing materials that release organic acids and tannins. Peat, Indian almond leaves, alder cones, and other botanicals slowly bind calcium and magnesium ions, reducing hardness slightly while tinting the water a warm, earthy brown.
These natural acids can also lower pH and reproduce the chemistry of blackwater habitats.
However, their effects are limited and unpredictable. They work best as refinements rather than foundations, ideal for fine-tuning chemistry and adding authenticity once the main hardness has already been reduced through dilution.
Safe Practice
Softening water should always be done gently. Large or sudden reductions in hardness can cause osmotic stress, disrupting the internal balance of fish and the stability of the filter.
Prepare and test new water separately before adding it to the aquarium.
When first transitioning an aquarium to softer water, take it slowly.
Gradual change is far safer than dramatic adjustment. Replace small portions of water over several weeks, allowing fish, plants, and bacteria to adapt with each step. Patience ensures stability and prevents the shock that can follow rapid shifts in chemistry.
As carbonate hardness (KH) is reduced, remember that pH stability also weakens.
KH acts as the water’s buffer, protecting against sudden swings in acidity. When KH falls below roughly 3–4 °dKH, pH can become far more reactive to biological processes such as respiration and decay. Regular monitoring and gentle maintenance help prevent fluctuations.
Evaporation presents another quiet challenge. As water evaporates, only pure H₂O leaves the system; minerals stay behind. Over time, this can cause hardness to climb. Always top up evaporated water with pure RO or deionised water rather than tap water. Doing so keeps both GH and KH steady between water changes.
Once your ideal blend is established, maintaining it becomes routine. Every water change becomes a repetition of the same ratio, keeping GH, KH, and pH steady and predictable. Over time, the system settles into a quiet rhythm, and both fish and plants adjust fully to their environment
Commercial Hardness Adjusters
Many products claim to raise or lower hardness with a few drops of liquid, promising instant results and minimal effort. They are marketed as convenient alternatives to mineral salts or reverse-osmosis blending, yet the reality is rarely so simple.
Liquid “GH boosters” typically contain a mixture of calcium, magnesium, and sometimes trace elements dissolved in acidic or chelating solutions to keep them in suspension. While they can increase hardness, the actual mineral composition and concentration are often undisclosed or inconsistent between batches. Because they are added directly to the aquarium, it is easy to overshoot the target and create sudden chemical shifts that stress fish and destabilise the biofilter.
“Water softeners” and “hardness reducers” are even more problematic. Some contain sodium salts that exchange calcium and magnesium ions for sodium, temporarily lowering GH readings but leaving the water chemically unbalanced. Others use phosphate-based binders that can fuel algal blooms or interfere with nutrient uptake in plants. A few rely on weak organic acids to dissolve carbonate buffers, producing a short-term drop in KH and pH that quickly rebounds once the acid is neutralised. In every case, the change is temporary, unpredictable, and often more disruptive than helpful.
For aquarists seeking lasting, measurable control, physical methods remain the gold standard. Using pure water to dilute hardness and mineral salts to rebuild it with precision allows chemistry to be adjusted gradually, tested reliably, and repeated consistently. These approaches work with the natural properties of water rather than against them
In short, no bottle can replace understanding. Stable hardness comes from knowledge, patience, and deliberate preparation, not from additives that promise instant results. When we learn to shape our water thoughtfully, every adjustment becomes an act of care rather than correction.
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