On March 7th, I bought three platies from Petco. I had never had problems before with disease, so I had stuck two of the platies (which were sunset platies) into my 26 gallon that had six White Cloud Mountain minnows and a ghost shrimp. The other platy (a red wagtail) was put in my sister's empty ten gallon. A few days later, both the wagtail and one of the sunsets had died. They both had been gasping at the surface with gills open for the last couple hours before dying. I hoped that that had been the last of it, but in a few more days then it killed one of my minnows. I had noticed the minnow gasping at the surface and had stuck her in the empty ten gallon with a dose of artemiss. The minnow died within an hour. I did daily water changes for a week, and when none of the fish got sick than I patted myself on the back. A month later (today), then I walked into my room and saw my last platy flouting around the tank, dead. She had one side of her gills that was red and inflamed. I've been doing daily water changes, my water tests fine, the only thing noteworthy is that my heater is to big for the tank, and until I get a new one, then the tank is at 80 degrees f. Instead of the much preferable 75. My other minnows are acting fine. Please help me; I don't want my whole tank to die off!