Planted Tank Guru
Join Date: Dec 2017
Location: Elk Grove, California
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
180 g. low tech w/ wild South American cichlids, corydoras eques, and African Congo riverine tetras.
60 g. low tech w/ F1 Alenquer pair /Stendker "Tefe" discus and wild Altum Angels
30 g. low tech w/ Wild Tucano tetras
30 g. low-tech African Biotope