Epsom is OK for long term use. It's aquarium salt that can cause damage if used for too long.
I wonder what his problem is as maracyn shouldn't work on columnaris because columnaris is a gram negative bacteria. Maracyn fights gram positive bacteria.
gram positive bacterial infections, fin and tail rot, popeye, body fungus
These antibiotics prevent protein synthesis and effect the cells ability to reproduce.
Further information can be found under the name Erythromycin
bacterial infections, intestinal flagellates
It was noted to be quite effective when the pH is neutral or slightly alkaline.