A Fish Amoxicillin is effective against a number of pathogenic bacteria associated with marine and freshwater ornamental fish diseases. Use For: Infected Eyes, Fin or Tail Rot, Skin Ulcerations, Pop-Eye, Columnaris Disease, and Gill Disease. Brands May Vary! Amoxicillin is a broad spectrum bactericidal antibiotic. It is effective against gram-positive and gram-negative bacteria.