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?