well there are all kinds of fish he could get,especially if hes not going with corals
check out angels, butterflys(can be touchy), wrasses, an eel, lionfish, im not sure if any type of stingray would fit in there.. ik there is one that gets a disk of around 12inches so maybe.., it really all depends on what he likes.. there are so many fish to choose from.. maybe just go on the site and look at all the fish pictures and write down all the ones he likes along with a rating 1 to 5 on how much he likes them and then pick his favorites as long as it would be a suitable home inside that size of tank with the other inhabitants u choose
btw i just remembered u said you had a clean up crew.. does your puffer not eat any? also take this into consideration when choosing fish..