I have to agree with the other people here on this. The grouping is by far not a grouping that should have been sold to you really.
I'm confused as to just what the stock is in by the first initial question. Are we to believe that the entire stock you listed is in the 30 or that you have a 30 you could isolate an aggressive fish to be in? I think in order to examine what is going on, it would be best to know just what the whole stock is being kept in now.
All that though, I'd have to say that if a group like that isn't in somthing around 150 and up, you're going to have a recurring problem. 125 isn't bad either, I'd go with at least a 6 foot span. It sounds like a boundry issue for starters, and likely, the tank they are in too small. Probably just a rehash again of what others have said, but you may find some benefit to looking at your species tendencies, to overall adult size, and looking at your tank footprint when looking at a mix.
