mystery disease again...

My tank is crashing fast. The first fish was sick about 2 months ago and since then I've lost about 7 more. Right now all I have left in my 75 is a pair of pearl gourami and a few platies. I've been really frustrated because I can't figure out what this is. This week I've done several massive water changes and I always do 25% a week so parameters are fine. Ammonia=0, nitrite=0, nitrate=10. Carbon in the filter, and Prime for water changes. The illness progresses like this: first stage is isolation and sitting on the bottom, then heavy breathing, then death. Some of them also get edema and bloating. Oddly enough, they all had normal appetite right up until the end. I've lost bettas, platies, and fancy goldfish to this same disease. I tried Tetracycline at half dose in the hospital tank and both times the fish died within minutes. Any ideas?

Well, everyone is doing much better now:clap: I did not lose any more fish and sx are now gone. The last few fish that died were white and grey. When I observed a gold colored fish showing signs of illness, I noticed what looked like grey dust and very tiny dots (smaller than ich). I blacked out the tank, added 1tbsp salt/gallon, raised the temp to 80, added air stone, put in a 24watt UV sterilizer (with low gph), and malachite green/formalin ick cure. I figured I need to hit it hard because so many fish already died. So my best guess is that is was some type of very strong velvet protozoan. Now that everyone is healthy I have to think about how to restock...