Pharmaceutical grade amoxicillin for your aquarium fish. Amoxicillin is a widely used antibiotic that is effective against some of the bacterial ailments that commonly afflict fishes, particularly gill diseases, and chondrococcus (which appears as a cottony growth around the mouth). Effective against gram-positive bacteria, some gram-negative bacteria will be resistant.
Overuse of antibiotics can be detrimental in fish as in humans; bacteria can become resistant or mutate, rendering antibiotics ineffective. Use antibiotics not as a cure-all but only when absolutely necessary to treat the infection at hand.
It is also a good idea to quarantine your afflicted fish in a smaller tank and treat that tank, while thoroughly cleaning your main tank and using UV sterilization to prevent recurrence of the offending bacteria. Do not use with invertebrates. When possible, mix antibiotics with fresh or frozen fish food for direct ingestion to avoid contamination of tank and other organisms.
Manufacturer's instructions: Add contents of one capsule per each 10 gallons aquarium water. Repeat in 24 hours. Change water between treatments. Treat from 5 to 10 days.