Fishbiotic Penicillin is a Pharmaceutical Grade Fish Antibiotic. Fish Penicillin which is bactericidal against gram-positive and certain gram-negative bacteria. It is used to control certain bacterial illnesses in fish, such as flexibacter infections, tail rot, and fin rot. They come in penicillin 250 mg and Fish penicillin 500 mg. Currently they can be purchased in 30 and 100 count bottles.