Prolonged salt/med use can also cause fin melt, is something I've recently learned.. he might not even have had rot to begin with.
Fresh, warm, conditioned water is best. If there's an underlying issue messing with his immune system, that'll become apparent in time. But yeah, try a few weeks just on the clean water, changes twice a week. If he's just got rot, then he ought to pick up soon and show signs of healing.
A bare-bottom tank won't stress him. My boy with chronic fin issues is in a bare-bottom tank with a ball of java moss and some floating wisteria to help him feel secure - he's pretty happy in there. No substrate = much easier cleaning, plus you can keep an eye on what he excretes, in case it's an underlying problem.