I've had a pair of firefish gobies in my 125G for a couple of years, and the space they typically inhabit is probably no more than 10 gallons. They're always in the same spot in the center of the tank... unless they happen to chase down a piece of food that drifts out of their neighborhood. I have a variety of fish in the tank, and they have no problem co-existing with any of them. They tend to be jumpers, so be sure to keep your tank covered.