the male was going into breeding age possibly so he'd be trying to court a female which accounts for the harassment, if theres no female he'll go after a male and chase him a little. But more over why do you have a Lake Malawi fish mixed with SE asian river and SA river fish? I really suggest you remove the malawi cichlid, all of them if you want to have an easy school. your idea with the rainbows is good, you just need to increade the population. Cories aren't hard to keep though I suppose you lab was possibly murdering things at night. You can keep a school but research. not all species of cories are compatable with EACHOTHER. Also Google, Reaseach, Crossrefrence.. play it safe so this will not happen again