I'm fairly new at fish keeping, but I've read a lot about the do's and dont's. A week ago I done a 30% water change and after filling the tank back up I lost my red tail shark the following day. Just 2 days ago (Saturday) I did another water change and that night I lost a gourami and yesterday one on my platy's died and then today when I got home from work my cory catfish is dieing. All fish have come to the top of the water as if their not getting enough air. I have a 75 gallon tank with 6 live plants and one of the long air stones and 2 filters. Please help!!!