Your tank isn't cycled? Do you change the filters too much? Ammonia should always be at 0, and maintained at 0 before adding any fish. Some just don't tolerate any ammonia at all, and others tolerate levels for a little bit before taking the plunge. Assuming internal infections are ruled out, I'd say the gourami died as a result of a year's worth of low levels of ammonia toxicity.

I wouldn't bother with stress coat, it takes up oxygen in the water. Dechlor is all you should need since the fish are showing disease. I quarentine for 8 weeks. One thing to watch is fish poop. If the fish are pooping white, then they for sure has an internal infection or parasite. When picking out fish in a store you can watch for this before choosing the fish. Poop patrol may sound funny, but its an efficient way to pick out healthy fish.