Tetracycline if you can issolate, if not maracyn one and two just to be on the safe side, good luck.
As tetracycline will wipe the bacteria colony out in the filter in the main tank.
Tetracycline
Manufacturer: Aquatronics
A broad-spectrum antibiotic used in the treatment of gram-positive and some gram-negative bacterial infections. For infections of fin and tail rot, frayed fins, popeye, inflamed gills, mouth and body open sores/ulcers, livebearer disease, dropsy, Columnaris, and secondary infections such as fungal. Specific for livebearers and goldfish. Aerate the aquarium well when using. Useful for the control of some common bacterial diseases, including Aeromonias and Pseudomonas Genera and the Mysobacterial group.
Maracyn
Manufacturer: Mardel
A broad-spectrum antibiotic for gram-positive bacterial infections. For infections of columnaris (body fungus), fin and tail rot, popeye, gill disease, and secondary infections.
Active ingredient: Erythromycin.
Maracyn–Two
Manufacturer: Mardel
A broad-spectrum antibiotic for internal or external gram negative bacterial infections which can even be absorbed through the skin. Effective treatment of fin and tail rot, popeye, gill disease, dropsy (swollen body, protruding scales), septicemia (bleeding or red streaks on the body), secondary and internal infections. Effective even when fish won't eat.
Active ingredient: Mincycline hydrochlor.