I think what he is currently eating plays into it a little. I trained a 9 inch one i use to have by holding the bag of feeder fish in front of his tank to the point where he was snapping at them through the glass. Then id bring him up to the top of the tank by moving the bag, then throwing a massivore pellet in. I think shrimp/krill is a luxury diet with a really great taste to it, and if i get another I'm not going to start feeding him that if i have the choice. Another one i had i got onto floating freeze dried krill, but no matter how many times i tried he wouldn't take to anything else, and i think it was because of the sweet taste of krill. However if he's already eating shrimp, stuffing it is the way i would go. I must say my datnoid, the first one i mentioned, didn't eat anything for about 3-4 weeks while i was trying to get him on pellets. If he's of decent size like you said 5-6 inches, don't be afraid to starve him a little. I also had a group of 6-7 frontosas in with him, and they were active eaters, eating pellets.