i suggest if your running carbons remove them from the tank.. start dosing the tank for the next few days with melafix this will at least reduce his stress .try to feed a few times per day when there are new to a tank they can be very shy eaters although you might have a tad of overfeed it will give him time enough to eat while your not looking ..try some emerald entrese thawed in a cup then dump into the tank .you can soak these in selcon and or garlic extreme to boost vit in it for him ..also try flakes and pellets a seaweed sheet for a tidbit to munch on.. as for the aggression from your unicorn they are the same family and do not normally play nice together