it's play sand. This sand is the most natural look I can find that isn't hard to clean when I do clean the tank. I would like to use a mix of play sand, mud and rocks but filteration maintenance would be a nightmare. So I just use the sand for substrate. Personnally, I think sand is easier top clean than rocks. The best tank to keep clean is a bare bottom. I went with that setup when I first started caring for RES. But about 5 yrs ago I decided to try play sand. Its has been my substrate of choice ever since.