unfortunately tbar the lfs
is completely wrong, i have a picasso trigger and a niger as well in my 125 and they do extremely well. the 90 is simply way too small for an undulated. i have also seen that triggers tend to become destructive and ignorant when in smaller tanks. you will def need a larger tank for when he gets bigger, or a possible re-home
(id buy him off ya)
in an attempt to rid him of his bad behavior, try taking him out and then completely re-arranging the tank...itll make him feel that hes in a new tank. Dont mean to come across mean, just that the tank size is prob playing a part in this aggressiveness. there are numerous triggers that get 15+ inches in length and need a ton of room for swimming, as you can see they dont stop moving.