I was terrified of that happening when I got into mbunas... The best advice I ever got was to start with more than you want and don't get attached as they will kill indiscriminately. I've had gentle species be the meanest in my tank, and different types of blue zebras get along with each other. My tank hasn't followed any "rules" beyond being busy and entertaining. It's so hard watching a well planned out tank do its own thing. Sorry about your fish...