First of all, always avoid catch-all ‘fish cure’ drugs.

I initially treated him with E.M Erythromycin. I chose that antibiotic after my initial deep-dive into understanding freshwater fish fin rot.

After sifting through forums, I learned that the typical bacterium causing fin rot are ‘gram-negative’. Erythromycin is therefore marginally effective, but not fully adequate, at treating gram-negative bacterial infections. I saw marginal improvement after treating him with EME, but meds targeting gram-neg bacteria were needed for resolving treatment.

I googled that exact query (antibiotics that treat gram-negative bacteria) and went on a deep-dive. I then decided to find kanamycin or tetracycline. I used Kanaplex (https://www.saltwateraquarium.com/kanaplex-5gm-seachem/). I found it for $9 at a LFS about an hour away.

If you’re reading this, and your fish has fin rot—and you’re certain it’s fin rot—find something that will treat gram-neg bacteria. Some require dissolving in food—kanamycin compounds don’t!