API Erythromycin

API Erythromycin is a prescription-strength erythromycin antibiotic formulated as a water-soluble powder for the treatment of common gram-positive bacterial diseases in freshwater aquariums. It is particularly effective against the bacteria responsible for fin rot, tail rot, and mouth fungus (columnaris), which are among the most frequently encountered bacterial infections in the hobby.

Erythromycin works by inhibiting bacterial protein synthesis, preventing bacteria from reproducing and allowing the fish's immune system to clear the infection. It is less aggressive toward biological filter bacteria than broad-spectrum aminoglycosides, though some disruption is still possible during a treatment course.

As with all antibiotic treatments, it is most effective when the underlying cause — poor water quality, overcrowding, or aggression — is also addressed alongside the treatment itself.