Methylene blue is one of the oldest aquarium medications and remains a useful tool for fungal infections, egg fungus prevention, mild antiparasitic treatment, and supportive care for fish suffering from ammonia or nitrite poisoning. It is a dark blue dye that binds to proteins and has multiple modes of action — it inhibits fungal growth, kills some external protozoans, and can help convert methemoglobin back to functional hemoglobin, which makes it useful in cases of nitrite poisoning. Methylene blue is an OTC aquarium product, not a prescription-grade veterinary medication. It stains silicone, sealant, plants, and decor blue, so most hobbyists use it in a hospital tank or as a dip rather than in the display.

Overview

Methylene blue has been used in aquarium and veterinary medicine for more than a century. In fish it is primarily used for:

  • Treating and preventing fungal infection on fish and eggs
  • Short antiparasitic dips
  • Supportive care during ammonia or nitrite poisoning (reduces methemoglobin formation)
  • Preventing fungal growth on newly laid eggs in breeding tanks

Because it stains heavily and can inhibit beneficial bacteria, methylene blue is usually not added to the main display. It is most often used as a bath in a separate container or in a dedicated hospital tank.

Uses

  • Saprolegnia and fungal infection of fish
  • Egg fungus prevention in breeding tanks
  • Mild external parasites (including ich at low effectiveness)
  • Emergency treatment for nitrite poisoning (methemoglobinemia)
  • Shipping bag additive to reduce stress and stabilize during transport
  • Supportive care for fish recovering from ammonia exposure

Dosage

  • Hospital tank dose: 1 teaspoon (5 mL) per 10 gallons of 2.3% methylene blue solution, or follow label
  • Short dip: A stronger concentration for 10 to 30 minutes in a separate container for heavy fungal loads
  • Egg treatment: Small amount added to a breeding tank or egg tumbler to prevent fungus on newly laid eggs
  • Nitrite emergency: Label dose in the affected tank or hospital tank to aid recovery

Always follow the product label for exact dosing.

Safety Considerations

  • Inhibits biological filter — Methylene blue is toxic to nitrifying bacteria. Do not use in your main display tank if you want to preserve your cycle. Always use in a hospital tank or as a dip.
  • Stains everything — Silicone, sealant, decor, substrate, and plants will be stained blue, often permanently. Use a bare hospital tank or separate container.
  • Invertebrates — Not safe for shrimp, snails, or corals
  • Plants — Will be stained and may be damaged
  • Scaleless fish — Generally tolerated at standard doses in hospital tanks
  • Fry and eggs — Commonly used safely on eggs and fry at reduced doses
  • User safety — Stains skin and clothing. Wear gloves when dosing.

Water Chemistry Effects

  • Turns water deep blue, reducing light penetration
  • Inhibits nitrifying bacteria — not for use in cycled display tanks
  • Activated carbon removes methylene blue effectively after treatment
  • No significant effect on pH, hardness, or salinity

How to Use

  1. Set up a bare-bottom hospital tank or separate dip container
  2. Add water from the main tank (for hospital use) or mix fresh treated water
  3. Dose methylene blue per label
  4. For dips, keep fish in the concentrated solution for 10 to 30 minutes with close monitoring
  5. For hospital tank treatment, keep fish for 3 to 5 days or as directed
  6. Return fish to the main tank (after acclimation) when treatment is complete
  7. Use activated carbon to remove residual dye from the hospital tank water

FAQs

Can I use methylene blue in my display tank? Not usually. It will stain everything and crash your biological filter. Use a hospital tank.

Is it safe for eggs? Yes, it is widely used to prevent fungus on fish eggs and is considered safe at proper dose.

Will it stain my fish? The dye may temporarily tint the fish, especially light-colored species. This fades over time.

Can I use it for ich? Methylene blue has some antiparasitic activity but is not as effective as Ich-X or dedicated ich treatments. It is sometimes used as a supportive treatment.

Does it help with nitrite poisoning? Yes, methylene blue can help convert methemoglobin back to functional hemoglobin, supporting fish suffering from nitrite exposure while the underlying water quality issue is addressed.

Is it invertebrate-safe? No. Do not use in tanks with shrimp, snails, or corals.