A tank that small is not reccomended for a beginer.The water quality can be very trickey to maintain.It can turn from fine to toxic in a matter of hours.But if you do it,I wouldn't put more than 10lbs of rock in there.You need to leave some room for the fish to swim...And you shouldn't put more than one or two SMALL fish in that small of a tank...A good hang on the back type powerfilter should work fine....