Many fish suffer from bacterial infections and there are many treatments available for such problems. Antibiotics are not normally available 'off the shelf' and are usually administered via injection by a vet or fish health professional.
Antibiotics are indiscriminate so they will harm both good and bad bacteria, therefore, its use must be considered carefully. Most bacterial treatments available in aquarium shops are not antibiotics in this sense; they are focused treatments designed to affect specific types of bacteria. It is always worth checking the box though; some of these treatments may affect oxygen levels or even kill some of the useful bacteria in the aquarium filter.