The other serious problem with adding medications to a cycled main tank is that you will wreck your cycle. When you kill the beneficial bacteria that make your water safe for your fish, you get spikes of ammonia and nitrite that will further weaken an already sick fish and ultimately kill all your fish. If you absolutely need to medicate, it's best to do it in a quarantine tank.
Large water changes with dechlorinator are imperative no matter what. They will not only give your fish pristine water for recovery, but also will remove any ammonia and nitrite that may be building up if you did damage your cycle. You will need to be testing water daily and might have to let hte cycle reestablish. This all assumes that your tank was cycled in the first place. If it was not, that was probably the cause of the problems, and you should read up on cycling immediately.
Is this a dwarf gourami? If so, there's a high chance that you are dealing with an incurable iridovirus. I avoid dwarf gouramis as a rule because so many of them are infected already when they are imported.
I'm sorry your fish is sick, and I hope he gets better.
