Methylene Blue in Fish Treatment: Egg Fungus and Nitrite Support
How methylene blue is used as an antifungal for fish eggs and as supportive treatment for nitrite poisoning, with sourced concentrations and biofilter and plant cautions.
Overview
Methylene blue is a thiazine dye (a phenothiazine derivative) sold as a dark powder that yields a deep blue solution in water. In fish health it is used mainly as an antifungal and antiseptic to protect fish eggs and as a supportive treatment for nitrite poisoning. It is a dye, so it stains silicone, decor, skin and other materials blue.
What it does
The Merck Veterinary Manual lists methylene blue among the common antifungal chemicals (with malachite green and acriflavine) used against Saprolegnia, the water mold that infects fish eggs and damaged tissue. Methylene blue is most commonly used to protect newly laid eggs from fungal infection. It is also a recognized treatment for methemoglobinemia: in the body it is reduced to leucomethylene blue, which converts oxidized methemoglobin back to functional hemoglobin, the basis of its supportive use in nitrite poisoning, often called brown blood disease.
Administration and dosing
Methylene blue is given as a bath or short dip, in a hospital or hatchery container rather than the display aquarium. Because published concentrations vary with the goal and the species, sourced figures are summarized below; egg-treatment safety is concentration-dependent, with lower concentrations generally protecting hatch better.
| Use | Concentration | Notes (source) |
|---|---|---|
| Egg disinfection (marine medaka, peer-reviewed) | 40, 80 or 160 mg/L for 1 hour | Hatch fell from 93% at 40 mg/L to 84% at 160 mg/L |
| Egg-treatment toxicity threshold (peer-reviewed) | 96-hour LC50 about 127 mg/L | Lower concentrations safer for embryos |
| Prolonged immersion, fish (peer-reviewed) | about 2 mg/L | Studied in goldfish over 21 days |
In the marine medaka study, methylene blue protected developing embryos better than formalin while still reducing microbial load, and lower concentrations preserved hatch rate. The egg-protection concentration should be chosen toward the lower, hatch-sparing end of the tested range and applied in clean water.
Safety, biofilter and plants
Methylene blue can negatively affect beneficial bacterial communities, including the nitrifying bacteria of a biological filter, so it is used in a separate hospital or hatchery tank rather than an established display aquarium to avoid disrupting the nitrogen cycle. As a dye it stains aquarium silicone, ornaments and equipment, and it is generally not used in planted display tanks. It is not approved for food fish.