Fish Metronidazole 250 mg is an antibiotic that exerts a bactericidal action on gram-positive and some gram-negative bacteria in fish. Useful to help control some common bacterial diseases in fish such as Aeromonas, Pseudomonas, and Mycobacterial (Gill diseases and Chondrococcus). Effective against anaerobic bacteria, a bacteria that only grows where there is no oxygen. Also used to control parasitic diseases such as Cryptocaryon irritans, Ichthyophthirius multifiliis, and Hexamita.