Substrate.. using sand as a substrate if fine... IF .. you keep your eye out for pool filter sand. Playground sand can and will compact. PFS on the other hand has been graded to a size of .45 - .55 mm which allows water to flow thru it, hence it's use in pool filters. Some of the PFS's will make your tank cloudy for a day or so if ya didn't rinse it extremely well. The brand I buy, "Lighthouse", can go in the tank without any rinsing and will clear up in an hour... instantly if I add the water slowly. Best of luck! Bill in Va.