Well in my humble opinion it appears he's being dominated, not sick. I bet if you moved him to another tank where he was the "alpha" male he'd be much happier. Male Gouramis like to be the most dominant fish in the tank. They're still okay when they aren't but why not try to make him happiest? I know you already said you can't have another tank.............but maybe you can set up a 10 gallon for him? I successfully kept a large male Powder Blue Dwarf with Fancy Guppies, Glowlights & a Snail in a 10 gallon. It's not the best size tank for him but far better than letting him get picked on isn't it?
At pet sores they typically keep 5-10 male Dwarf G's in one tank that can't be larger than 20 gallons and it appears to work, but that's because there are so many of them in such a small place that the alpha male can't just pick on one fish. Then someone comes along and buys 2 or 3 of them and put them in their own tank and all hell breaks loose, even if the tank is significantly larger than the one you got them from. Happens all the time with these beautiful fish unfortunately.
At pet sores they typically keep 5-10 male Dwarf G's in one tank that can't be larger than 20 gallons and it appears to work, but that's because there are so many of them in such a small place that the alpha male can't just pick on one fish. Then someone comes along and buys 2 or 3 of them and put them in their own tank and all hell breaks loose, even if the tank is significantly larger than the one you got them from. Happens all the time with these beautiful fish unfortunately.
I will also buy aquarium salt then to add to his new tank (dw I have kept it running so its already cycled - has been set up for like 2 years).
