I used old carpet. It's usually free, it's tougher than most liners, and it won't be seen covered with sand and rock. Some of mine wasn't even covered, and is not visible at all now under the water after being covered for 10 months. It's covered with silt and plankton growth. It holds plankton growth very well. I think it's a fine re-purpose of something that usually ends up in a landfill. If it's a bright color as some of mine was, turn it upside down. It's not hard on feet either.