sandy gravel for sure. You could also have a mix with really pea sized gravel in there. My Puffer's love both for two distinct reasons. The sand because they like to dig themselves in it, and the gravel because they like to scoop it up in their mouths and trim their teeth
It's WICKED cool watching them have their entire bodies dug into the sand with just their heads popping out. Then you toss some food in there, and BAM, they jump right out. Incredible personalities. Too bad they're so mean to other fish
mine is in medium grave and like what Ben said, they occasionally "bite" the pieces of gravel
and this would probably help in keeping its teeth trimmed.
yeah, at that size, he's more likely to dig himself into the sand. They do it less when they're bigger, because well, what fish is gonna mess with an 18 inch fish with HUGE teeth