fish mox forte Aquarium Amoxicillin 500mg exerts a bactericidal action on gram-positive and some gram-negative bacteria. Amoxicillin is a broad spectrum bacterial antibiotic. It is effective against a number of pathogentic bacteria associated with marine and freshwater ornamental fish diseases. Can use for infected eyes, pop-eye, fin or tail rot, skin ulcerations, columnaris disease, and gill disease.