Re: So, you want a reason to not mix species?
Whether or not there is aggression in mixed species tanks is a competely seperate issue from intra-species aggression. It's sort of like saying, "all of the astronauts that have visited the moon have been white American men, so women and non-white, non-American persons would be unable to survive on the moon". Your observed behaviour does not logically transfer to your suggestion that mixed species tanks would only increase aggression.
Richard Lynch (Giant Orange, Regina, Citronella, Suriname Cobalt, Powderblue, Alanis, Azureus, Patricia, Oyapok, Green Sip, tinc Nominat, Leucomelas).
If you raise poison dart frogs, you can't say that you'd never hurt a fly!!