Methylene Blue for Fish

Fact checked by Emily EstepReviewed by Nick Saint-ErneFact checked by Emily EstepReviewed by Nick Saint-Erne

Methylene Blue (chemically known as methylthioninium chloride) is a cationic dye, redox indicator, and photosensitizer. In aquaculture, it serves as an anti-fungal and anti-parasitic drug and has commonly been used to treat fish eggs to ensure they are not lost to fungal overgrowth.

Methylene Blue is an aquarium-safe disinfectant and can also be used for the treatment of ammonia and nitrite poisoning.

Methylene Blue Uses

Disorders that Methylene Blue is used to treat include the following:

• Nitrite Poisoning: Fish gasp for breath, have tan or brown gills, and rapid breathing movement known as gilling.

• Ammonia Poisoning: Fish gasp for breath, have red or purple gills, and are lethargic—may lay on the bottom, with red streaks on the body or fins.

• Ichthyophthirius multifiliis: Small white spots on the skin; fish scratches against objects.

• O?dinium pillularis: Also known as Velvet, this is actually caused by a parasite.

• Swim Bladder Disorder: Fish struggle to maintain proper position, float upside down, and swim with their tails higher than their heads.

• Egg Fungus: Serves as a safe and mild disinfectant for fish eggs.

• Fish Stress: Prophylactic treatment of fish under stress, usually used during shipping.

Safe Applications

Methylene Blue can be used with crustaceans, including crabs, shrimp, and snails, but should be introduced carefully. It will damage live plants at doses over 4 milligrams per liter and should be used with caution in such cases, ensuring that plants are exposed for only limited periods of time.

Methylene Blue should not be used concurrently with Erythromycin or Tetracycline antibiotics. After using a Redox-reducing type of water conditioner (which includes most water conditioners), wait for 30 minutes before adding Methylene Blue. Discontinue the use of carbon media in filters while treating, as carbon will remove the Methylene Blue.

How to Treat Fungus on Fish Eggs

For prevention or treatment of fungus on fish eggs, follow these instructions for treating your aquarium with Methylene Blue:

1. Remove carbon from the filter and continue to operate with mechanical filter media.

2. Add 1 teaspoon (5 milliliters) of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 parts per million (ppm). For increased concentrations, add approximately 1/3 teaspoon (1.64 milliliters) per 10 gallons for each required 1 ppm increase.

3. Treatment should continue daily until three days past the free-swimming stage.

External Parasitic Protozoans

If your aquarium is affected by a fungus or by external parasitic protozoans, apply Methylene Blue using the following steps:

1. Remove the carbon filter and continue to operate with mechanical filter media throughout the treatment period.

2. Add 1 teaspoon (5 milliliters) of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 ppm. Continue the treatment for three to five days.

3. Make a water change and replace the filter carbon after the treatment.

Nitrite or Cyanide Poisoning

As an aid in reversing nitrite (NO₂?) or cyanide (CN?) poisoning of marine or freshwater fish, use Methylene Blue in the following application:

1. Remove the carbon filter and continue to operate with mechanical filter media throughout the treatment period.

2. Add 1 teaspoon (5 milliliters) of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 ppm. Continue the treatment for three to five days.

3. Make a water change and replace the filter carbon after the treatment.

Use as a Dip

You can also use Methylene Blue as a dip for treating fish with fungal infections, those affected by external protozoan parasites, or those suffering from cyanide poisoning.

1. Prepare a nonmetallic container of sufficient size to contain the fish to be treated by adding water similar to the original aquarium water.

2. Add 5 teaspoons (24.65 milliliters) per 3 gallons of water. This produces a concentration of 50 ppm. It is not recommended that the concentration be increased beyond 50 ppm.

3. Place the fish to be treated in this solution for no longer than 10 seconds.

4. Return fish to the original aquarium.

Products

Fish Vet Methylene Blue

Dosing Instructions: 1.25-ounce dropper bottle dosing instructions: Add one drop per gallon. Perform a 25% water change after three to five days and replace carbon. If used as an egg fungal preventative, continue treatment two to three days after fish are free-swimming.

Powdered concentrate dosing instructions: Remove the top and slowly add water to the top of the bottle. Replace the cap tightly and shake gently to mix (we recommend using reverse osmosis or distilled water). Add 1 teaspoon per 10 gallons. Perform a 25% water change after three to five days and replace carbon. If used as an egg fungal preventative continue treatment two to three days after fish are free-swimming.

Discontinue carbon filtration but keep all other filtration running during treatment. Replace carbon after treatment.

Kordon/Oasis Methylene Blue for Aquarium

Follow the manufacturer's directions for use: The following procedures are suggested for freshwater and marine aquariums and ponds. Methylene Blue can be absorbed by porous materials such as rock, coral, and wood. The product is best used in bare aquariums or ponds, especially if they are new. Methylene Blue may permanently color the silicone sealant in aquariums. At the conclusion of all treatments, a partial or complete water change should be made, and activated carbon replaced in the filter.

Methylene Blue is effective against superficial fungal infections of fish. The drug may be used as an alternative to Malachite Green for the control of fungus. Methylene Blue is safe for use with fish eggs and fry for the prevention of fungal infections. As a secondary use, it is effective against some external protozoans, such as Ichthyophthirius (Ich), Chilodonella, and Costia.

Methylene Blue converts methemoglobin to the normal oxygen-carrying component of fish blood, hemoglobin, allowing it to be used in the treatment of known cyanide and nitrite poisoning of aquarium fishes. It has been suggested that newly arrived marine fish placed in Methylene Blue can have their survival rate increased as Methylene Blue aids in the reversal of nitrite and/or cyanide poisoning.

Read Next: The Importance of Using Malachite Green in Aquariums

Read the original article on The Spruce Pets.