We have a 25 gal. tank with mostly mollies. The water has always tested perfect for GH, KH, PH, and shows no ammonia. We had a guppy disappear overnight a few weeks ago. I took the tank apart (we use the cuttings from our 75 gal. planted to decorate) washed everything, cleaned the gravel, changed the filter media and have done several water changes. All fish appear to be healthy. They eat and are active. Then when we get up in the morning or come home from work there will be a dead fish. When I got home yesterday it was a pregnant mollie dead. This morning a female puffer mollie. Only one fish out of five we lost showed any signs of illness. That was a large male sailfin mollie that developed a white fuzz on his left side. We set up our hospital tank and treated him with melafix but he only lasted 3 days. We hate to lose our fish and this has always been our easiest to maintain tank. Any suggestions would be appreciated. Thanks!