Having had a couple of cups of coffee I now remember the antibiotic was amoxyl and I still can't remember if that is the same as bactrim. most antibiotics are used at 1/100,000 or 10ppm but amoxyl could be used up to 5/100,000 or 50ppm. The bacteria usually enters through a wound but if the fish are stressed they may have had a hard time from other parasites which could leave lesions.