When I moved my Dwarf and Opaline into the same tank, they seemed to go thru some mating posturing. (I truely do not know much about group dynamics for these fish.) The opaline then became very agressive and would chase the dwarf back up into his corner. 4" cube of space all of his own... in a 29 gal
. He developed a sore close to his mouth and the first several spines under his belly became blackened. As soon as I had the 6 gal
free I moved him in. The sore and spines slowly faded away like they were just battle bruises. I too tried a general anti, but no change really happened. He continued to eat and move around normally.
Now it could be that I was truely clueless and that luck favors the bold, small children and ships named Enterprise....but all signs are gone.
I hope that it just resolves itself for you as well.
Oh, and to everyone else... if I was just lucky and this is some weird black plague of gourami 8O let me know!