Any Idea What This Is?

Have you guys ever heard of "Panalog?"
It's a topical antibiotic cream made for farm livestock, but it works absolute wonders on fish. A little dab'll do ya fer shure in emergencies. In some areas you can get it in farm supply stores, but in others you have to go to a vet. The stuff works wonders, soaking into the infection immediately when directly applied and stopping it cold within hours. It isn't the most convenient thing to use by any means, of course, but if you ever have a case where hours count, consider it if you can get it. This particular fish wasn't in that bad a shape, but I'm sure we've all seen much worse and probably lost a few specimens while waiting for the more conventional methods to kick in. I just figured I'd mention it while I was thinking about it.

When I worked at a native species hatchery we had an outbreak of columnaris in one species of fish when they were first brought in. Columnaris attacks when the fish is under a great deal of stress. An additional factor can be when it's water parameters change greatly in temp, hardness, pH, etc. For future preventative measures I would not suggest "spring cleaning" your tank like that again. By removing all the water and scrubbing everything down, you pretty much removed all your beneficial bacteria which kept your tank in check (I agree with you, teleost). This probably sent your tank in to a minicycle. It also caused a great deal of stress on your fish being moved back and forth, espcially when he was returned to his home tank which was basically now an uncycled tank. As far as the salt, you really don't need it unless it's native waters have some degree of salinity. Salt should generally be reserved for times like these to help with stress and treatment.