Metronidazole is useful for the treatment of certain protozoan caused diseases of fresh water and marine aquarium fish.

It is primarily indicated for the control of diseases caused by Hexamita and Spironucleus (the suspected causative agents of "hole-in-the-head" and "head-and lateral-line-erosion" syndromes in fish).

A secondary use for Metronidazole is for the control and treatment of Ichthyophthiriasis ("ich")