It just depends on the system and the fish involved. None of the fish he has require a lot of swimming space. Most of them are not particularly territorial either, with the exception of the tomato clown. I would keep an close eye on that one though. If his water parameters stay healthy and none of the critters are being stressed out by their tankmates then I don't see a problem with it. Of course I had 17 fish in my 55gal for well over a year without any problems. :wink: