cant keep cts our water is way too hard. Never had fin rot either but if mine are ill I put them in clean treated conditioned water with a heater, and a smidge of aqua salt. Seems to do the trick no gravel or ornaments in there either
If the finrot is the sort where the fins go black or grey, then fray and fall off, use melafix, salt and warm, clean water. If it is the sort where white lines appear on the fins and work toward the body, this is bacterial and much more serious, especially if it is affecting the pectoral (swimming) fins. Antibiotics are usually curative - use anything you can get that says it is an antibiotics. Tetracycline, oxytetracycline (oxytet) chloramphenicol, erythromycin/streptomycin/kanamycin - anything of that sort will cure the finrot.