Updated: Nov 16, 2020
Methylene blue (also known as methylthioninium chloride) is a longstanding treatment for fish which has been used by generations of aquarists. It is effective against a range of fungal and bacterial infections and has proved itself to be a very useful weapon in the arsenal of pufferfish keepers around the world.
Methylene blue is definitely a treatment that we recommend every aquarist has at their disposal, just in case it is ever needed. Typically, when you do need it, you need it straight away, so it is a good idea to have it in your aquarium cabinet at all times. For emergencies, you may be able to order it online for next-day delivery using Amazon Prime.
Fungus and pufferfish
Methylene blue has been used in the aquarium hobby for decades, being very popular with breeders who use it to stave away fungus from fish eggs. Methylene blue is especially effective against fungus and this is where it has really proved its worth with keepers of pufferfish. Fungus, can be found in almost every aquarium on Earth, feeding on decaying organic matter. These fungi rarely attack healthy fish, but they are opportunistic feeders and can quickly take advantage of an injured or sick tank inhabitant and gain a foothold in scratches, scrapes and other infection sites, where they begin to spread as it eats away at flesh.
Pufferfish succumb very quickly to fungal infections, so it is important to act fast and hit the infection hard! Methylene blue baths can be the weapon which wins the war and ultimately saves the fish.
What you will need for a Methylene Blue Bath:
Aquarium strength Methylene Blue/methylthioninium chloride We recommend King British Methylene Blue, or Kordon® Methylene Blue.
Syringe or a pipette
A non-metallic and food safe container which holds at least 9 litres (2.38 US Gallons) of water.
For King British Methylene Blue, use 1ml to every 9 litres.
For Kordon® Methylene Blue, use 1ml to every 8.6 litres.
Carrying out the methylene blue bath in five easy steps
Put 9 litres of tank water into a container. Always use tank water, so the fish doesn't experience parameter or temperature shock.
We have found that clear storage boxes made from non-toxic plastic work best for Methylene Blue baths, because they allow you to observe the fish through the transparent material.
Because this will need to be repeated several times, you may find it useful to draw a line at the water level of 9 litres, on the outside of the container with a Sharpie marker. This will make future measuring easier and faster.
Bigger containers will be necessary for larger fish and the amount of methylene blue used will need to be increased accordingly.
Using a pipet or a syringe, put the correct amount of the methylene blue into the 9 litres of water (read 'Dosages' above) and gently stir until the solution has been thoroughly mixed into the water. Always put the methylene blue in before the fish.
Place a jug into the tank until it is full to the top. Net the pufferfish and put the jug underneath the fish. Lift the jug and the net out of the water at the same time, so the fish isn't lifted out of the water and has an prolonged opportunity to inflate with air.
Take the net and jug and place it as close to the methylene blue bath as possible so you can transfer the fish quickly.
Quickly and carefully transfer the fish into the methylene blue bath.
Leave the fish in the solution for 60 minutes.
It is best to place the Methylene Blue bath somewhere quiet where the fish won't be disturbed or startled. If your container came with a lid then you can use the lid to prevent the fish from jumping out of the solution, providing that a gap is left to allow for oxygen.
After 60 minutes, use the jug and the net to transfer the fish back to the tank.
Remember to minimise the length of time the fish is out of water as much as possible.
The transfer needs to be done as quickly as possible.
Pour the methylene blue water away, down a suitable drain (e.g toilet, sink).
For fungal infections, we recommend two baths per day, for one hour each, for a minimum of seven days or until the infection has gone. We also recommended carrying out big (at least 50%) water changes on the aquarium, while the fish is in the methylene blue bath, to keep the water as clean as possible and reduce the amount of fungal spores in the aquarium. Always make sure that the water parameters match.
It is important not to add methylene blue into the aquarium. Some sources say this is fine to do, but methylene blue can destroy filter organisms, which could then expose your fish to lethal levels of ammonia and/or nitrite.
Pufferfish health information given on this site is not intended to act as or replace the advice of a certified veterinary professional. If your pufferfish is experiencing a medical emergency, contact an experienced aquatic veterinarian immediately.
This guide contains affiliate links, which means that Pufferfish Enthusiasts Worldwide receives a small commission from the products that our readers buy through us. It doesn’t cost you any extra, and the proceeds go toward keeping Pufferfish Enthusiasts Worldwide’s care guides free.
Thank you for your support!