Probably one of nicest backgrounds I've had in my tank was a mold of a rock formation that was on an Island on the seashore. I noticed the really interesting background in one of our public aquariums. What it consisted was plastic mold material that could be poured on a formation and then released after a period of time. I think I first applied a release compound first on the formation.

It came out really nice only next time I'm going to spread different pigments to look like rock layers like shale. Once you have the mold off you can trim it to match your tank. Was a little pricy since it took a few times to get it right..

Had caves and everything. Did take a few days to find exactly what I wanted to copy. If anyone is interested I can research what I used as it was very difficult to get the public aquarium people to tell me what they did. After bending many ears someone told me their secret. I will occasionally check this post or you can leave me a message on the "New to the forum page\ if you are interested. :)