Methylene blue or methylthioninium chloride is a heterocyclic aromatic chemical compound with the chemical formula     C 16 H 18 N 3 SCl . Methylene blue is a green and odorless solid that dissolves in water and creates a blue solution. This basic color is used in various fields. The most common use of methylene blue is as an oxidation and reduction indicator in analytical chemistry. This solution is seen in blue color in oxidizing environment and is colorless in front of reducing agent. Also, biologists use methylene blue to take tissue samples and detect nucleic acids.

In the past, methylene blue has been used to treat methemoglobinemia such as cyanide poisoning. Today, it is used in medicine to treat various diseases and disorders such as Alzheimer’s, kidney stones, schizophrenia and herpes infections. Also, dentists use methylene blue to detect areas with microbial dental plaque.

In the field of aquatics, one of the ways to prevent fish eggs from being contaminated by harmful bacteria and fungi in the water is to add methylene blue to the water. For this purpose, methylene blue is used in aquariums to treat and prevent fungal infections. This substance protects fish eggs against bacterial agents and is also used for local treatment and fish wounds. The presence of this substance in water disinfects the aquarium and reduces the prevalence of bacterial and fungal infections of fish in fresh water.

Medicines containing methylene blue treat external parasites of fish and poisoning caused by nitrite in water. Also, the presence of methylene blue is very effective for preventing the presence of bacterial and fungal agents. The low toxicity of methylene blue against all kinds of fish has made this substance still in use. On the other hand, the presence of methylene blue in aquariums gives it a blue color, which reduces the intensity of light and improves oxygen supply.

Tips about using methylene blue in aquariums:

• It is not recommended to use this substance before spawning of fish because it can reduce the fertility of fish.
• The use of methylene blue in aquariums, in addition to destroying harmful bacteria, also disrupts the cycle of useful bacteria, which should be noted.
• In case of using special aquariums for plant growth, the presence of methylene blue is not recommended because it causes the leaves to turn blue or the plants to die.