You should remove the fish.
Almost every DG for sale is male and they do not like each other despite what some people want to think...
Likely the disease ,if not just DGI due to poor fish breeding by the Asian fish farms is a disease that will become contagious likely caused by stress from the other DG...
9 out 10 times one kills the other and people fail to see why...
Sorry about your fish and keep a close eye on the others.
In most cases when the physical symptoms are this clear to see the issue is to great to be treated or cured.
TB [mycobacterium marinum] has no cure ...
Same for DGI...
http://www.aces.edu/dept/fisheries/e...20gouramis.pdf