What about Tim? Do they ever invite Tim to play?In my experience, Colorados and Chicagos, and normal rbta don't get along well together. One or the other will almost always melt unless there's a large amount of carbon being used, this suggests allelopathy at play.
Doesn't happen in a tank full of the same strain, so... I guess they're all 'Hi Gerald, Hello Gerald, Hey I know you, You're Gerald, right?" Until you put a Frank in the mix.... Gerald hates Frank.