Hi. im glad to hear your fish is still alive.
As far as you being a bad fish keeper don't you belive that.
. 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.