I realize this is just the type of info your are looking for. Be careful buying a goby that relays on the nutrients in the sand to survive. Especially in a tank your size. It will sift until it has depleted all its food source, then starve and die. That being said, jtgrimes's suggestion of a yellow watchman and a pistol shrimp are a great addition for a tank your size. Another great selection is a conch. I have one in my 40b, and it does great job keeping the sand bed clean.