You could try adding more or add more caves like others suggested. If you have places to hide already you could try re-arranging the tank that might ease the aggression over territories
I recently got 3 cobue and they have adjusted very well to the tank. Temps running 81 n pH a little over 8. they get along well with my other mbuna and the dominant cobue is the boss of the tank
I had 2 Stealth heaters, in one tank the plastic bubbled up and cracked without it running dry and poisoned the water, other one continues to work til this day.
teacups are usually retic's, 90 would work for a little while but eventually you will need a bigger tank. Retics are usually fragile too, one of the harder to keep rays.
Sorry to hear about your loss, i recently lost a ray that seemed otherwise healthy and active. Never stopped eating or got skinny. Truly lost as to what happened to the little guy, as i had him over a year.