Going to use the black extra fine sand. And for the person who said that sand is messy, : during the bi-weekly 'vaccuming' I normally turn off the filters then wait 2 min. or so for most stuff to settle, then turn back on asap. And weekly I take apart the water filter bottom part to clean out anything (hair, cat fur, pieces of plants, etc.) that's in the compartment with the impeller. No problems so far.