I like to use the small rocks. about 1/2 to 1" in diameter. Sand gets stirred up too easily and can possibly get sucked into the filter and wreak havoc with the pump. The smaller rocks are still easy enough to use the vacuum-type siphon and work through maybe once a month to clean out. cannot do that with larger rocks. I size filters for at least 2x the actual size of the tank, but sounds like yours is doing a good job for now.