a dark green dye used as a stain, an antiseptic, a chemical indicator, and an antidote in cyanide poisoning

3,9-bisdimethylaminophenazothionium chloride trihydrate, C16H18N3SCl.3H2O, a thiazine dye and redox indicator.

A very common medication sold both as the foundation of various cures and as Methylene Blue. This is one of the few medications that are sold branded and in solution as a product in its own right. In this form, usually as a 1% or 2% preparation, it can be safely used by the pond keeper. It is commonly used for fungal infections and some external protozoan parasites. It is generally added to the treatment tank water. Methylene Blue is deactivated by organic matter and is damaging to aquatic plants.

Methylene blue is a heterocyclic aromatic chemical compound with molecular formula: C16H18ClN3S. It has many uses in a range of different fields, such as biology or chemistry.