Hi. im glad to hear your fish is still alive.
As far as you being a bad fish keeper don't you belive that.
Things happen. Your tank looks very good and you were smart enough to see your fish was sick, that shows you care enough about your fish to seek help. I would keep your fish isolated for at least a week after the medicine. I know a bucket is not a great place, Maybe you could purchase a 10 or 15
gal. tank to keep as a hospital tank they don't have to be decorated a bare bottom and something to hide if the fish needs to. Keep an eye on your fish I think there is a section on the Maracyn box or paper that goes over this. Also the chart inside the box is well worth hanging onto as there is a diagram that helps diagnose fish ailments. I would also run tests on your tank & a water change with carbon. Maybe this will show what happened to cause this. Keep us posted and good luck.
EMS503