Methylene blue

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This is a dye used in chemistry as a blue staining agent or dye. It has also been used for years in the aquarium hobby as it is also useful at destroying fungus, bacteria and parasites.

  • It is probably most often used as a anti-fungus agent when trying to hatch fish eggs.
  • It is usually supplied as a powder and is relatively safe if used correctly.
  • It is also useful for the treatment of known nitrite or cyanide poisoning of aquarium fishes. This dye has the ability to increase in the animals' red blood cells the amount of haemoglobin (oxygen carrying cells) by transforming methemoglobin (iron-carrying protein) to oxyhemoglobin (oxygen carrying protein).
    If you suspect a new fish has been kept in levels of nitrite water, then dose the fish whilst in the bag or in a quarantine tank. Do not dose the main tank.


Standard treatment of fish or other aquatic animals

You will need to make up a 1% solution in distilled water (typically use 1g dissolved in 100ml of water). Then add 2ml of this solution for every 10 litres of quarantine tank water every 2-3 days. If the colour fades over this time add more dye until the colour is restored. Never let the water colour get more than a light-medium blue in colour however.

Standard treatment of fish eggs

Dosage is around 2mg/litre. But one approach is to add a drop at a time of a 1-2% solution until the water turns a light-medium blue with the egs still visible.
It is recommended that by the time the egss start to hatch that the dye will have dissipated by itself or via water changes. The dye will degrade naturally over time depending on the amount of Dissolved Inorganic Carbon (DIC) in the water.

  • Please note this chemical kills bacteria so will seriously damage the bacteria in your Filter system.
  • This chemical will dye ornaments, your skin and the silicon seals in your aquarium.
  • This dye is toxic to plants and an irritant to human skin.
  • Avoid skin contact.
  • Avoid contact with clothing.
  • Avoid breathing the powder or dust.

Commercial prepared solutions

Virtually all aquatic companies sell Methylene blue as a pre-prepared solution ready to be placed into a quarantine tank. Though few if any tell you the percentage of concentration used.