I have an established 30 gallon freshwater tank where all species of fish that I have thrive (platy, glass catfish, tetra) EXCEPT my Gourami. I've had 3 randomly go lethargic and die over the past 3 months. They were the neon blue variety. They just start hanging out near the heater and quit eating. Next day...dead on the bottom. Are they touchy, or what?