any particular reason?
i'm asking more about keeping halfbeaks in the small tank. i periodically add guppies to the tank (it's well filtered and oxygenated - several mangroves and a ton of algae growth in addition to the mechanical filter) and as the species don't compete terribly for space eveyone does fine in the tank. (i add the guppies when the females are ripe - the gobies love the babies)