I have a female molly who is very aggressive towards my platies and a female dalmation molly that was pushy towards the platies and a male black molly that was trying to nip at one of my platies fins. I have the mollies in a separate tank now and they are doing fine together. The dalmation molly died this morning, she was a new fish that I just got a few days ago. After i got her is when the aggression started. Could they have gotten sick and would that cause the aggression?