Those antibiotics commonly available at fish shops include tetracycline and triple sulfa. Illegally, nitro furans and kanamycin are sometimes purchased/imported.
Before we dive right into the answer on which is the best antibiotic to use to treat such fish, have a look at this picture.
In this diagram is a table of the antibiotic sensitivities of a said fish/aquatic bacteria. ‘S’ stands for sensitive and in theory, it should be effective. ‘R’ stands for resistant and in theory, will have negligible effect on the pathogen.
Whenever I choose to test fish with antibiotics, it’s a good idea to run cultures and an antibiotic sensitivity panel to determine:
1. What bacteria you have?
2. Is it a primary pathogen or secondary invader?
3. Which antibiotic would have the greatest effect if necessary?
In this picture, you may notice that not all antibiotics will work every time. It’ll depend on the bacteria and on the strain. It may also depend on what you’ve used in the past. Consider evidenced based medicine, rather than a ‘quick fix’ recommended by a friend.
In another picture, the antibiotic sensitivity panel is not looking as bad. The discs are antibiotic impregnated cards and the space seen around them (areas of inhibited bacterial growth) is measured and compared with the standards to check if they are effective.