I actually used a marine sand that had (in very tiny letters) "suitable for FW aquariums" on the bottom. I mixed it with playsand that was in the original tank (not this one), so now it's a mix of very fine sand and the marine stuff. I don't know exactly what it is, think it's crushed shells etc. I thought I'd have issues with my pH but it stays at a steady 8.0 so no problems there.