Thats a 15 gallon correct?
You could have ~15 Bumble Bee Gobies
While small they are awesome fish and very territorial so they chase each other around the tank but never cause injury.
A bit of a pain to feed since they won't take flake but mine take frozen blood worms, frozen glass worms, live brine and live black worms.
You could even have some halfbeaks since BBGs stay on the bottom of the tank.