A marble goby and water cow are two different fish. Marbles get quite a bit larger in the wild, largest I've seen in captivity isn't think 18in now. Water cows, aren't as greedy and are more diurnal, where as marbles are nocturnal. Marbles are more greedy ime, both will try to eat whatever fits in there mouth, marbles are more aggressive and also have larger teeth so hold there own better, if it's a water cow, fish like Oscars are good, not aggressive, and to big to eat, if it's a marble, I say be a little more selective cause they're more willing to try a tank mate as a snack ime. Both slow fish.