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Methylene Blue for Fish

By | Last Updated: 23rd February 2023

What is methylene blue and what is it used for

Methylene blue, otherwise known as methylthioninium chloride [6], is a dye that is also used as a medication for conditions like urinary tract infections and cyanide poisoning. It is also used as an anti-malarial agent [1].

Can aquarium fish take it

In aquaculture, methylene blue is used primarily to ensure that fish eggs are not affected by fungal infections [3]. It is also effective at treating various conditions in fishes like ammonia and nitrite poisoning [1]. Protozoan infections like chilodonella, costia, and ichthyophthirius (ich) can also be treated with methylene blue [5]. Although, for ich (characterized by small white spots, and the fish scratching against the objects within the aquarium) [1], a combination of Malachite Green and formaldehyde is considered to be more efficient [6].

Quick Information

Methylene Blue for Fish

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Generic Name: Methylene blue

Type: Anti-parasitic, anti-fungal

Who can take it: Humans, fish

Is it approved by FDA: Not for use on fish

Availability: Prescription not needed

Available Forms: Powder

How does methylene blue work in fish

Methylene blue works by interfering with the oxygen circulation within the body at a cellular level. Methemoglobin is a component of blood that makes it less efficient in carrying oxygen throughout the body. However, methemoglobin exists in small amounts under normal circumstances. Methylene blue increases this amount, thus further hindering the oxygen circulation; as a result, it eliminates the infection-causing pathogens by preventing them from spreading [6, 7].

How much methylene blue to give

As this medication is indiscriminate against all bacteria, including the good ones living within the original aquarium, it is usually recommended to separate the affected fishes into a separate quarantine fish tank during treatment [4].

Treatment in a fish tank: If you are using methylene blue as a general detoxifier against nitrite, ammonia, or cyanide poisoning, for general disease prevention or to keep fish eggs safe from fungal infections, typically 1 teaspoon of methylene blue is added for every 10 gallons (37.8 liters) of water, or 10 drops per gallon (3.78 liters). For a thicker concentration, you may add around 1/3 teaspoon per 10 gallons [5, 1]. These are the commonly recommended measurements, but it is safe to talk to a fish vet before application.

Dip treatment: You will need a large non-metallic container for this one. Usually, 5 teaspoons of the powder is added for every 3 gallons of water, and the affected fish is placed in the solution for a maximum of 10 seconds before it is placed back into its aquarium [1]. But, make sure to consult a fish vet or an ornamental fish expert to know how a dip treatment is done as it might turn harmful for your fish if anything goes wrong.

During the treatment, it is recommended to keep the aquarium filtration running, but only after removing the carbon filters. Once the treatment period is over, the aquarium water is changed, and the carbon filters reapplied [1].

Side effects

As long as it is given as per the recommendations, it is usually safe. Fishes may even tolerate moderately higher dosages without any adverse effects [5].

In case of an overdose

Methylene blue can be removed by activated carbon filtration, so replacing the carbon filters in your aquarium is the first thing you can do. Also, make a complete water change in case you have put in too much of the medication [1]. Additionally, it is also recommended to move the fishes to a separate aquarium or water tank as soon as possible.

Contraindications: When not to give

The medication is not given if a fish has:

  • Parasitic infections like velvet disease (Oodinium) and fluke diseases
  • Bacterial infections
  • Moderate to severe fungal infections

It is also not to be used as a net disinfectant, or as a sterilizer [5].

The product should not be used with aquariums with biological filtration, as methylene blue interrupts the regular biological processes of nitrifying filter bacteria [2]. Even the plant growth within the aquarium may be hampered [5].


    1. Methylene Blue – TheSpruce.com
    2. Methylene Blue: A Chemical for Treating Aquarium Fish – WorldOfChemicals.com
    3. Methylene Blue – DrsFosterSmith.com
    4. How to Treat an Aquarium with Methylene Blue – Animals.mom.me
    5. ┬áMethylene Blue – Kordon.com
    6. What Is Methylene Blue? – EverydayHealth.com
    7. Treating sick aquarium fish with dips and baths – Algone.com

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