Looks like it could be Uronema. One symptom are red bands that show up on the fish's flanks. Watch the fish's respiration rate - if it begins to increase in a day or two, that might be what this is. I hope not, because it is very difficult to treat. It is a protozoan that is capable of living between the cells in the fish's body - virtually impossible to get any medications in there. it is not however, an obligate parasite, so its presence does not always mean the other fish will come down with it....you can isolate it from most aquariums, nobody knows why some fish get infected while most don't
If it were my fish, I'd catch it up and take a tiny bit of the tissue from the lesion and look at it under a microscope at about 60 to 80x - look for moving protozoans.
One treatment I've experimented with is chloroquine at 10ppm...but it hasn't worked well on this intracellular Uronema.
Jay