Methylene Blue Overdose, Possible To Use Too Much In a Tank?

Last Updated on November 24, 2021 by cmoarz

Methylene blue, also known as methylthioninium chloride, and Methylene blue dye. If you have fish then you’ve undoubtedly heard of this medication before on various youtube channels and recommendations as a treatment for various fish ailments.

But is it really safe? Can methylene blue kill fish if it’s overused? What kind of Methylene blue side effects are there? This article is going to dive right into all of that including dosing, and overdosing, as well as answer a few common myths.

What is Methylene Blue Used For In A Fish Tank?

Methylene blue treatment is primarily a fungicidal that can be used to treat fungal infections fish, in the water column, and on various other organisms in your tank.

Malachite Green is the most common fungicide, but MB may be used as an alternative to malachite green in cases where the fish to be treated are known to be sensitive. You can read more about Malachite Green here.

Methylene blue is a safe and effective treatment for fungal infections in fish eggs and fry. It’s also used to treat fish with external protozoans like ich, chilodonella, and costia as a secondary function.

It’s especially important for certain species of fish eggs and fry that are very susceptible to fungal growth (See: Corydoras eggs).

It’s also been known to help with nitrate poisoning, ammonia poisoning, Multifilis, and Pulularis, and swim bladder disorder.

Prolonged exposure to Methylene Blue will hurt plants specifically. It will also dye everything blue, including aquarium decorations.

It also shouldn’t be mixed with Erythromycin, or Tetracycline (antibiotics). You should also wait a solid 30 minutes to 1 hour after using a water conditioner before adding MB.

For these reasons, Methylene Blue is recommended for hospital, quarantine, and fry tanks. Not your main planted tank, Unless you don’t care what happens to the plants or everything having a blue tint to it! (It even stains the silicone seals!)

Also, Avoid using any active carbon filter media. It will pull the methylene blue treatment out of the tank quite effectively.

Methylene Blue Usage in Fish Tank, How Much Is Too Much?

Methylene blue comes in many different forms, such as a liquid and in powder form, all of which have slightly different instructions. The most popular products people use are Kordon, Biopharm, and a few others.

Unfortunately, dosing instructions can be tricky depending on the product you choose. This is because of varying qualities, quantities of ingredients, and potencies.

Methylene Blue is used for many things outside of fish care and thus has many different use cases. When buying MB, be sure to get something made for an aquarium so you can get the proper dosing instructions. Kordon for example has instructions that state:

The following instructions are taken directly from Kordon’s website and are intended for use only with Kordon products.

Warning: Activated carbon filtration removes Methylene Blue. It will also be absorbed by porous materials like rock, coral, and wood. Methylene Blue is most effective in bare aquariums and ponds, especially if they are new. In aquariums with silicone sealant, methylene blue may be absorbed by the material. At the conclusion of all therapies, a portion or total water change should be performed, and activated carbon in the filter should be replenished.

Prevention or treatment of fungus on fish eggs:
(a) Remove carbon from the filter and continue to operate with mechanical filter media.
(b) Add 1 teaspoon of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 ppm. For increased concentrations, add approximately 1/3 teaspoon (1.64 ml) per 10 gallons for each required 1 ppm increase.
(c) Only one application is needed. Treatment should continue for 3 days past free swimming stage or for livebearers 2 days after birth.

Prevention or treatment of fungus or external parasitic protozoans:
(a) Remove carbon filter and continue to operate with mechanical filter media throughout the treatment period.
(b) Add 1 teaspoon of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 ppm. Continue the treatment for 3 to 5 days.
(c) Make a water change as noted and replace the filter carbon at the conclusion of the treatment.

As an aid in the reversal of nitrite (NO2-) or cyanide (CN-) poisoning of marine and freshwater aquarium fishes:
(a) Remove carbon filter and continue to operate with mechanical filter media throughout the treatment period.
(b) Add 1 teaspoon of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 ppm. Continue the treatment for 3 to 5 days.
(c) Make a water change as noted and replace the filter carbon at the conclusion of the treatment. parasite protozoans.”

For use as a dip for treatment of fungus or external parasitic protozoans and cyanide poisoning:
(a) Prepare a nonmetallic container of sufficient size to contain the fish to be treated by adding water similar to the original aquarium.
(b) Add 5 teaspoons (24.65 ml) per 3 gallons of water. This produces a concentration of 50 ppm. It is not recommended that the concentration be increased beyond 50 ppm.
(c) Place fishes to be treated in this solution for no longer than 10 seconds.
(d) Return fish to original aquarium.

For products such as Biopharm which don’t have specific use instructions for aquarium use, You will need to pull out your calculator and adjust the portions to match the 1% vs 2.303% that’s in Kordon. Then dose like you would with Kordon’s dosage and treatment instructions.

When used correctly, Methylene Blue Dye is completely safe and easy to use!

So Why Do My Fish Die After I Use It?

That’s a great question. Given how safe methylene blue is, there are a few reasons this could have happened.

You overdosed: Methylene blue is a great treatment for fungus and ich in the tank. But it’s not so great for your fishies if you accidentally give them too much and cause them to overdose.

To remedy this in the future, Consider applying the MB directly to the fish itself and not the water column (when possible). It’s very difficult if not impossible to overdose a fish tank doing this, and you won’t hurt the beneficial bacterial in the tank. Plants will also be safe.

Your fish were sensitive to the chemicals: This is rarer because of how easy MB is on most fish (as opposed to malachite green) and is usually used as an alternative. But it’s not impossible the fish you have just happened to be extra sensitive to the dye.

MB is usually considered safe for everything from fish, mollusks, shrimp, snails, etc. But it’s not unheard of for some species to be extra sensitive.

It’s also possible that the MB is not the right option for you given your tank set up (ex: planted aquariums, live rock).

If this is your first time using methylene blue then I would suggest next time you try aga again but with the direct application rather than water column dosing.

Your biofilter was damaged: It’s not unheard of for MB to damage the biofilter in your tank if you have a particularly sensitive one.

If this is the case, you may need to buy some enzyme supplements (bottled bacteria) like API quickstart or Fritz Aquatics and perform huge water changes afterward (to remedy).

You should know though that it takes time for the biofilter to recover and therefore future treatments with chemicals (even small doses) should be done with caution to avoid further damage.

If your biofilter was indeed damaged by the MB then there is a good chance your fish died of an ammonia or nitrite spike because the bacteria that normally processes these things have died off.

Once again I recommend direct application to fish wounds rather than dosing the water column as this will prevent much potential damage to plants and beneficial bacteria. These are called fish dips and are extremely effective.

Removing Methylene Blue From Aquarium Water

Removing Methylene blue from aquarium water is pretty straightforward. A combination of frequent water changes and an active carbon filter will remove most of it. But if you want to do it faster, here are some tips:

Do not reuse the treated water in anything, it should be disposed of. It’s not good for plants in your garden for example.

To remove MB from the water, perform a large (90%) water change and then use activated carbon in your filter to absorb it. If you can do both at once instead of waiting for one or the other go ahead! But either way works.

You may also need to alter the placement of your filter inside the tank/canister/sump. Moving the activated carbon closer to the water surface may make it absorb MB faster.

Keep in mind, If you used it in your regular tank, Many things may be dyed blue. That doesn’t mean there’s any left in the water column, but things may remain blue permanently.

This is especially true with decorations, silicone seals, and biological media that are not cleaned between treatments. The colored MB may stain them permanently.

Certain plastics may also become dyed blue, such as the plastic used on C02 injectors that aren’t glass. The same for C02 bubbler ceramics membranes.

How Long Can Fish Stay In Methylene Blue

Methylene blue treatment generally lasts for 3 to 5 days. Be sure to check the instructions on the bottle you are using since it should have this information.


Owner of and also owner of actual Aquarium Gravel believe it or not! ;). Setting up beautiful aquarium sceneries and habitats since I was very young. Enjoy!