Thanks! Yes, that is play sand on the bottom. I don't have problems with them getting out because they can't climb the walls, and nothing goes from the top to the bottom of the tank, so no escaping for them!
@Twilight Storm: They grow to about 4-5 inches. I have kept them with bettas before, but they do like to nip fins. I think they would do better with danios and other small, fast schooling fish.
The middle left one is a T shape PVC pipe so they can go underground. They love it!