As for treated wood, make sure you buy fence planks and not deck material. Cedar does get treated with a process called Wolmanizing. This is a salt-based preservative with some other chemicals in it that probably would not be the best idea to eat off of! You can usually tell treated lumber by the incisor marks on it with look like rows of 1/2" slits. This machine perforates the wood surface about 3/4" to allow the chemicals to penetrate to about 1" deep, thus ensuring a good treatment. Fence planks generally are raw, some are kiln dried, but no chemicals.

I plan to tell everyone that the secret to my fish is to let the planks catch fire! That way it looks like I meant it to happen. Boy, it sure turned out good!! The thin planks warped pretty good, some 3/4" planks should stay pretty flat.