metronidazole for fish

Quick Answer Metronidazole treats internal flagellate parasites (Hexamita/Spironucleus, the cause of hole-in-the-head disease and bloat) and some anaerobic bacterial infections in fish. It doesn’t treat fungus, external parasites, worms, or external bacterial issues like fin rot. Medicated food works best if the fish is still eating (Hexamita lives in the gut); a water bath treatment