Wash them a little bit in dechlorinated water in case there's anything on them, then set a small piece in the tank. Your water should change color and gain a yellowish brown tint...that's normal, expected, and desired.
cheers hope this works as i have tried everything else, if not i will try him in a tank on his own, he can't see chilli on the other side of the tank but he knows he's there, maybe he's intimidated