Wooden Airstones can be pretty expensive if you use a lot of them. I make my own from Basswood. I buy a block about 3x2x12 from the local hardware or hobby store for $5 and can make about 50 of them. I bought a pair of Coralife Wooden Airstones with the screw in nipples and use these nipples for my homemade stones. I believe from reading that Limewood is better, but it is VERY hard to find.