Did someone say the fish had columnaris? I missed that?
The fish has had an injury ( either another fish or an object in tank) because as you said, it was there one evening and gone the next morning). It also has become secondarily infected because of indications from fish behavior.
Unless other fish have lost or eroding fins ( in this case I would say it is moving away from injury into something else) or been injured at the same time as this fish and are expressing similar symptoms- why would other fish be sick and need treating? It is the injury itself that has allowed a secondary bacterial infection to weaken the fish to begin with.
I would recommend getting book on fish-keeping, basic water chemistry, and disease before you get more fish.
One of the best: The Manual of Fish Health
I think he is referring to my post on this thread Overnight Dorsal Fin Collapse And Discoloration On Pearl Gourami - Please Help! - Aquarium Advice - Aquarium Forum Community
where I say "That doesn't look like ich to me. It looks like a bacterial infection like columnaris. It's possible he rubbed his fin on something and that allowed the infection to set in. I have had something similar on my rainbow fish. I turned the heat up to 82 and added 1 teaspoon of aquarium salt per 5 gallons. Sadly one rainbow didn't make it but the other healed up just fine." I guess he read that as "this is columnaris" but that is not what I was saying.