dead molly

Hello all, I have multiple tanks (2x20 gallon) and (2x10 gallon), and in one of the twenty gallons I would like to add another Balloon Belly Molly. I have tried adding 3 over the course of the last couple months, but all the new ones die within about a week. The tank has 2 mature female ballon...

So yesterday I came home to a dead molly and I don't see any signs of illness.  If anyone knows anything, Id love answers! I really fear for my other fish Thanks!!!!!!!!!!!!!!!!! http://gyazo.com/0b97bec832ee05e72e3e5d1387d30093   Temp: 80 Ammonia: 0 Nitrite: 0.3 Nitrate: 0 PH: 7.2