I have (had) 2 all white mollies (I think). I fed the tank and watched all of them eat and watch for signs if illness as I have learned to do with my pond koi. I went to put shoes on to go out and my big molly (male I think) was upside down at the bottom of the tank gasping. I scooped him out and froze him in a little bit of tank water for a quick death but have no idea why he died when 2 minutes earlier he was eating like usual. My fish are all fine and levels are perfect. Any ideas?