The Pufferfish Keeper's Guide to Thiaminase
- Macauley Sykes
- 7 days ago
- 14 min read
Updated: 5 days ago
If you keep pufferfish, chances are you’ve come across the word thiaminase in discussions about diet.
It’s one of those terms that often pops up in conversations and care guides, usually accompanied by warnings, half-truths, or debates that leave new and experienced keepers alike scratching their heads.
Thiaminase has long been a concern in other corners of animal husbandry. Keepers of piscivorous birds, reptiles, and even marine mammals have had to reckon with it for decades because of the role it plays in vitamin B1 (thiamine) deficiency. But in the fishkeeping world, especially within the pufferfish community, awareness of thiaminase has lagged behind.
As a result, it remains one of the most common questions we hear: What exactly is thiaminase, and how does it affect my fish?
That’s why this article exists. Much of the existing guidance on thiaminase is scattered, contradictory, or framed in ways that don’t really help the everyday keeper make practical decisions. Some sources present long lists of “safe” and “unsafe” foods, while others downplay the risks altogether. The reality, as we’ll see, is more nuanced.
Our aim here is not just to explain what thiaminase is, but to place it in context: why it matters for pufferfish, how it fits into the bigger picture of diet and ecology, and most importantly, how you can approach feeding in a way that keeps your fish healthy without getting lost in fear or misinformation.
At Pufferfish Enthusiasts Worldwide, we believe that good husbandry starts with curiosity and a willingness to learn from nature. Thiaminase is a complex subject, but it doesn’t need to be intimidating. By looking at the science, considering the real-world experiences of keepers, and paying attention to what puffers eat in the wild, we can bring clarity to an issue that has confused keepers for far too long.
This guide is part of that mission. Whether you’re brand new to puffers or a seasoned aquarist looking to deepen your understanding, we hope this article provides both reassurance and inspiration. You’ll not only learn what thiaminase is and why it matters, but also gain a clearer sense of how to plan diets that are varied, naturalistic, and sustainable for the long-term health of your fish.
What is Thiamin?
Before we can understand thiaminase, it helps to first understand thiamine itself. Also known as vitamin B1, thiamine is an essential nutrient for almost all animals, including fish. It plays a central role in energy metabolism, allowing the body to release energy from carbohydrates, and it is vital for the proper functioning of the nervous system, muscles, and internal organs. Without it, even the most basic processes of life begin to falter.
Unlike some vitamins, thiamine cannot be manufactured or stored by the body in significant amounts. It has to be replenished regularly through diet. In the wild, fish achieve this automatically because their varied, seasonal feeding habits naturally provide what they need.
In aquaria, however, the responsibility falls on us as keepers to ensure those nutritional needs are met.
What is thiaminase?
Now that we know what thiamine is and why it matters, we can turn to the enzyme that threatens it: thiaminase.
Thiaminases are enzymes found in a surprisingly wide range of organisms and food sources. Their action is simple but devastating: they cleave the thiamine molecule, breaking it into pieces that the body can no longer use. Once thiamine has been destroyed in this way, it cannot be repaired or recycled. The result is a slow but steady drain of vitamin B1 from the system, something that, if left unchecked, will eventually undermine the health of the fish.
Researchers recognise two main types of thiaminase:
Thiaminase I – the most common form. This is the one aquarists need to worry about, because it is found in many of the very foods we feed: certain fish, shellfish, and even in some plants and bacteria. Thiaminase I is the enzyme most responsible for dietary thiamine loss in both wild and captive animals.
Thiaminase II – a less common form, mainly produced by certain bacteria. While important in microbiological studies, it plays a much smaller role in the diets of our fish.
For years, thiaminase was something aquarists hardly considered. It was seen as a problem for wildlife biologists studying salmon in the Great Lakes, or for zookeepers feeding piscivorous birds and reptiles. Only more recently has the connection been made clear for fishkeepers: the very same enzyme that devastated wild fish populations and caused neurological problems in penguins or snakes can also affect our puffers when we build their diets around the wrong foods.
Symptoms of a thiamin deficiency
A deficiency of thiamine doesn’t just cause one or two obvious problems. It can manifest in a wide spectrum of ways, and if left unchecked, it can ultimately be fatal.
What makes thiamine deficiency particularly tricky for keepers is how subtle and varied the signs can be. The symptoms often overlap with those of other health issues, making it hard to pin down the true cause. A fish that is losing weight, swimming oddly, or refusing food might be suffering from poor water quality, internal parasites, stress, or, less obviously, a lack of thiamine.
Among pufferfish keepers, the following signs are the most commonly reported in suspected thiamine-deficient fish: Growth and condition
Poor growth
Progressive weight loss
General weakness
Muscle wasting (atrophy)
Digestive and abdominal signs
Loss of appetite
Constipation or digestive issues
Abdominal swelling
Abdominal haemorrhage
Neurological and sensory signs
Loss of equilibrium and difficulty swimming
Convulsions and seizures
Blindness
Paralysis
Other neurological impairments
Nerve damage
Organ and systemic signs
Damage to, or outright failure of, internal organs
Reduced immunity (immunosuppression)
Reproductive problems, including the inability to spawn
The sheer variety of symptoms is why this deficiency is sometimes referred to as a “great imitator.” It doesn’t present as a single, neat syndrome, but rather as a patchwork of issues that can easily be mistaken for unrelated diseases. This is also why community discussions and shared experiences have been so valuable. When one keeper notices neurological problems, another reports reproductive failure, and a third sees appetite loss, it is easy to dismiss these as separate issues. But when those stories are shared, patterns emerge, and the role of thiamine becomes clearer.
Foods that contain thiaminase
Thiaminase shows up in many of the foods that pufferfish keepers commonly use, which helps explain why thiamine deficiency is such a persistent issue in the hobby.
Unfortunately, there is no definitive list of which species do and do not contain thiaminase. The science is incomplete, patchy across regions, and sometimes contradictory. Even within a single species, thiaminase levels can vary with season, location, or even between different tissues (for example, the viscera often carry higher activity than the muscle).
What we can say with confidence is that thiaminase has been documented across a wide range of prey groups:
Fish – especially members of the carp and minnow family (Cyprinidae), along with alewife, smelt, and other small forage fishes.
Bivalves – clams, mussels, and scallops (though oysters appear to lack thiaminase).
Gastropods – some snails and limpets.
Cephalopods – including squid and octopus.
Crustaceans – crabs, lobsters, crayfish, prawns, shrimp, krill, and barnacles.
Among the foods most frequently offered to puffers that are known to contain thiaminase are:
Fish from the family Cyprinidae (e.g., carps, minnows, barbs, barbels)
Certain prawns (e.g., Caramote prawn Melicertus kerathurus)
Various shrimp species (e.g., Macrobrachium rosenbergii and Penaeus spp)
Mussels, particularly zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena bugensis)
Various clams (e.g., Veneridae family)
Species-level insights
Mussels
Dreissenid mussels – Zebra (D. polymorpha) and quagga (D. bugensis) mussels consistently show very high thiaminase activity, in some cases 5–100 times higher than forage fish like alewife. Levels vary by season (highest in spring), location, and depth.
Blue mussel (Mytilus edulis) – Activity has been detected, but results are inconsistent: some studies find thiaminase, others do not. This may reflect differences in methodology or which tissues were tested.
Clams
Hard clams tested in mid-20th-century feeding studies showed significant thiaminase activity, enough to destroy measurable amounts of dietary thiamine.
Shrimp
In the aquarium world, shrimp (particularly Penaeus spp., such as tiger shrimp) are widely assumed to be thiaminase-positive. However, peer-reviewed, species-specific assays are lacking. The available evidence is anecdotal or generalised across crustaceans, rather than conclusive.
Why Thiaminase-Rich Foods Became Hobby Staples
It’s fair to ask: if we know thiaminase can be harmful, why are foods like clams, mussels, prawns, shrimp, and small feeder fish still so commonly fed to puffers? The answer lies less in what puffers truly need, and more in the realities of the aquarium hobby.
1. Misunderstandings of natural diets
For years, puffers (especially larger species) have been casually labelled as “molluscivores,” and the label stuck. This led many keepers to build diets around clams and mussels, sometimes exclusively. In reality, wild diets are far more diverse: some puffers are primarily insectivores, others opportunists, and even the big river species eat a wide variety of invertebrates. The oversimplification of “puffers eat snails and shellfish” has unintentionally driven many diets towards foods that carry a high thiaminase burden.
2. Price and availability
Mussels, clams, prawns, and shrimp are widely available in supermarkets at low cost. They’re easy to buy in bulk, easy to freeze, and easy to portion out. By contrast, more naturalistic food items, such as insects or varied invertebrates, are harder to source and often more expensive. For keepers on a budget, the convenience of supermarket seafood is difficult to resist, even if it comes with nutritional compromises.
3. Keeper convenience
Shellfish and frozen shrimp are not only cheap but also tidy. They don’t require culturing, collecting, or complicated preparation. A bag of frozen mussels or prawns can last weeks and feed multiple fish. By comparison, maintaining a supply of live foods or seeking out species-specific alternatives requires more effort, time, and in some cases specialist knowledge. Convenience often wins out, particularly for new keepers who want a straightforward feeding routine.
4. Tradition and habit
Once certain foods became standard in the hobby, they were passed down in care sheets, books, and forum posts as “the right way” to feed puffers. Over time, these normalised diets built heavily on clams, mussels, and shrimp, even though the science and field studies suggest these foods were never a good stand-in for the natural diets of most species.
Preventing a Thiamin Deficiency
Preventing thiamine deficiency is always better than trying to treat it once your pufferfish is already unwell. If a fish’s diet is made up largely, or entirely, of thiaminase-containing foods, deficiency becomes highly likely over time. This is not speculation but a straightforward biochemical process: thiaminase breaks down vitamin B1 faster than the animal can replenish it. As thiamine reserves are slowly depleted, the fish develops subtle health problems that can eventually progress to serious, sometimes irreversible, damage.
Some aquarists attempt to manage this risk by setting limits, for example, suggesting that no more than 20% of the overall diet should consist of thiaminase-positive foods. While this guideline is widely circulated, it is best treated with caution. The scientific literature shows that thiaminase activity fluctuates greatly: between species, between populations of the same species, by season, and even between different tissues of the same prey animal. For instance, the viscera of a small fish can carry far higher activity than its muscle, and the thiaminase load of a bivalve may shift depending on the time of year. What appears “safe” in one context can be unsafe in another.
Because of this uncertainty, at Pufferfish Enthusiasts Worldwide we recommend a more cautious stance: over the long term, the safest approach is to minimise or avoid thiaminase-rich foods altogether. This is not about restricting variety; quite the opposite. It is about avoiding a hidden dietary hazard that often goes unnoticed until it causes harm, and focusing instead on providing a rich, varied diet that mirrors what these fish actually eat in nature.
This perspective ties directly into one of our core principles: replicate the natural diet of each pufferfish species as closely as possible. The issue of thiaminase arises largely because puffers are so often kept on unnatural, narrow diets in captivity; clam, mussels, and endless cubes of prawn, or goldfish as a staple feeder. In the wild, puffers do not eat this way. They feed opportunistically across a wide menu, taking advantage of what their environment provides. They rarely, if ever, subsist on the repetitive, thiaminase-rich diets that cause problems in aquaria.
Replicating the Natural Diets of Our Fish
Rather than focusing only on what not to feed, it’s useful to look at how natural diets guide us toward safer choices. Each species of pufferfish has its own ecological niche, and when we replicate their natural diet as closely as possible, we often reduce thiaminase exposure by default.
This principle applies across the board: by studying what puffers actually eat in the wild, we can select captive diets that are both healthier and lower in thiaminase. In practice, this means providing a diverse rotation of foods that match the animal’s natural ecology, rather than relying on high-thiaminase foods as staples.
At Pufferfish Enthusiasts Worldwide, we’ve built this principle into our resources. Our individual care guides provide species-specific breakdowns of recommended foods, drawn from both the scientific literature and the collective experience of our community. By leaning on these guides and focusing on replicating nature, keepers can sidestep the pitfalls of thiaminase-rich diets and instead provide the variety, balance, and enrichment that puffers truly need.
For example, the Amazon puffer (Colomesus asellus) is often mislabelled in the hobby as a molluscivore, when gut-content studies of wild populations reveal it is primarily an insectivore, feeding heavily on aquatic insects and larvae. Insects, unlike clams, mussels, or feeder fish, carry little to no thiaminase burden.
This is why Pufferfish Enthusiasts Worldwide recommends various livefoods for the Amazon Puffer, including snails, pinhead crickets, woodlice, aphids, small earthworms (wisps), and several different types of worms, as well as various insect-based prepared foods such as Repashy Grubpie and Fluval Bugbites.
The Pea Puffer (Carinotetraodon travancoricus) shows a similar pattern: in nature, it thrives on a wide variety of insect larvae and micro-crustaceans.
Even the larger species, such as the Fahaka (Tetraodon lineatus) and the Mbu (Tetraodon mbu), are not narrowly molluscivorous in their rivers. Wild studies show that Fahaka display an ontogenetic shift: juveniles feed mostly on small crustaceans and insect larvae, medium-sized fish, soft snails and worms, and large adults use their jaw strength to crack heavy-shelled molluscs. The Mbu, ranging across the Congo basin and Lake Tanganyika, is a benthic opportunist, feeding on snails, freshwater crabs, crayfish, worms, insects, and occasional fish. In both species, molluscs form only part of a much broader menu.
In captivity, however, they are often restricted to raw mussels, prawns, and clams. This not only narrows the diet and increases thiaminase exposure, but also denies them the natural variety their metabolism is adapted for. Offering a diverse rotation of snails, crustaceans, worms, insects and insect-based foods better replicates their ecology, supports long-term health, and still provides the hard-shelled prey needed to control beak growth.
When we take our cues from the natural feeding ecology of these fish, the problem of thiaminase largely solves itself. By replicating the diversity, seasonality, and balance of wild diets, we both avoid the hidden risks of deficiency and unlock a more rewarding husbandry experience. Puffers fed this way are not only healthier, but also more engaged, displaying natural foraging behaviours that make them endlessly fascinating to keep.
This is also where our community shines. Many of the best breakthroughs in puffer husbandry come from dedicated keepers who go digging into field studies, ecological surveys, and scientific literature to better understand what their fish eat in the wild. When those findings are shared, everyone benefits: we gain practical feeding tips, new food sources to try, and a deeper appreciation of these remarkable fish. In this sense, following nature doesn’t just protect our puffers; it strengthens the knowledge base of the hobby itself.
Preventing thiamine deficiency, then, isn’t simply about avoiding a problem. It’s about aligning husbandry with ecology, feeding our puffers in a way that reflects their evolutionary history, and in doing so, setting them up for long, healthy, and vibrant lives
Treating a Thiamin Deficiency
The encouraging news is that thiamine deficiency can often be reversed if it is recognised and addressed early. Because thiamine is used rapidly by the body, restoring it through diet can sometimes bring about noticeable improvements in a matter of days. Appetite may return, activity levels may rise, and in young fish, growth can resume.
Even so, there are limits to recovery: fish that experience a deficiency during their developmental stages may regain condition, but the growth they missed out on often cannot be fully made up, leaving them permanently smaller than they might have been.
When cases progress further, the outlook becomes more complicated. Veterinary treatment, such as high-dose supplementation or injectable thiamine, can sometimes stabilise fish that are already showing neurological or systemic signs. Some individuals respond well, regaining normal function and living on without obvious problems. Others improve only temporarily, with lingering weaknesses that eventually resurface. And in the most advanced cases, where nervous tissue or internal organs have already been damaged, supplementation may slow or halt further decline but cannot undo the harm that has been done.
This variability is what makes prevention so important. Once thiamine levels fall too low, the difference between recovery and decline often depends on how quickly the deficiency is caught and how severely the fish has been affected. Early action can make the difference between a full return to health and a lingering vulnerability that never quite resolves.
For keepers, this underscores a simple truth: treatment can work, but it is no substitute for prevention. A diet that reflects the fish’s natural feeding ecology not only avoids the risks of deficiency but also spares the fish (and the keeper) the uncertainty of trying to reverse damage that may already be permanent.
Frequently asked questions
Q: Can I just feed thiaminase foods occasionally?
A: Feeding small amounts occasionally is less risky than making them the bulk of the diet. But because thiaminase levels vary by species, season, and tissue type, it’s impossible to set a universal “safe” percentage. Our advice is to minimise or avoid them long-term, and instead focus on replicating your puffer’s natural diet.
Q: Why do so many keepers feed clams, mussels, and shrimp if they can be risky?
A: Mostly because of convenience, cost, and tradition. These foods are cheap, easy to buy, and have long been recommended as “puffer staples.” Unfortunately, this doesn’t reflect the diverse natural diets puffers eat in the wild, and that’s where the problem lies.
Q: Can adding a thiamine (Vitamin B1) supplement to food counteract the thiaminase in that food?
A: Not reliably. Thiaminase acts by breaking down thiamine before the fish can absorb it. If the enzyme is present in a food item, then any thiamine in that food (including what you’ve added as a supplement) can be destroyed before it provides any benefit. In other words, sprinkling or soaking vitamins onto a thiaminase-rich food won’t make that particular item “safe.”
This isn’t just theory. When we reached out to the manufacturers of two of the best-selling aquarium vitamin supplements, they confirmed that their products cannot be relied upon to overcome thiaminase activity in the foods themselves. Supplements have their place, but they are not a free pass for feeding foods that are otherwise unsuitable.
Q: Does cooking destroy thiaminase?
A: Yes, heat will inactivate thiaminase, but the exact time and temperature needed depend on the food item, its size, and how it is prepared. For example, thorough boiling or baking is generally effective, but a quick blanch may not always be enough, especially for larger pieces of fish or shellfish.
It’s also worth noting that thiamine itself is heat-sensitive. Prolonged or very intense cooking can reduce the natural vitamin B1 content of a food, even as the thiaminase enzyme is destroyed. This means cooking can make a risky food safer by eliminating the enzyme, but it doesn’t necessarily make that food a strong source of thiamine on its own.
Q: Is thiaminase only found in saltwater (marine) animals?
A: No. Thiaminase occurs in both freshwater and marine species. It has been documented in a wide range of organisms, from freshwater cyprinids (carps and minnows) and dreissenid mussels (zebra and quagga) to marine fish like anchovy and smelt, as well as various clams, prawns, and other invertebrates.
Because it is so widely distributed, the presence of thiaminase is not linked to whether an animal lives in freshwater or saltwater; it depends on the species itself. This is one of the reasons why clear, species-level data is so important, and why keepers need to look beyond simple “freshwater vs. marine” assumptions when planning diets.
Q: Does freezing food destroy thiaminase?
A: No. Freezing does not inactivate thiaminase. The enzyme remains active in frozen foods and will continue to break down thiamine in that tissue, even while stored. This means that frozen clams, mussels, prawns, or thiaminase-positive fish still carry the same risk as when they were fresh.
It’s also worth remembering that thawing can leach out water-soluble vitamins, including thiamine, further reducing the nutritional value of the food. Freezing is excellent for preservation and convenience, but it does not make thiaminase-rich foods any safer from a vitamin B1 perspective.
Q: If my fish shows symptoms, can thiamine deficiency be treated?
A: Early cases can often be reversed by removing thiaminase-rich foods and restoring a balanced diet. In severe cases, veterinary intervention with concentrated thiamine supplements may be needed. However, nerve and organ damage caused by long-term deficiency is usually permanent. Prevention is always the safer path.
Q: What should I feed instead?
A: The best guide is nature itself. Different puffers have different diets; some are insectivores, some are opportunists, some mix crustaceans and molluscs. Our individual care guides provide species-specific food suggestions based on both wild ecology and long-term keeper experience. By following nature, you avoid the thiaminase trap and give your puffers the balanced nutrition they need.
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