I am interested in building a rock background with built-in caves. But my question is, is there a concern if water does not move through the caves? It seems to me there will be a lot of dead water in the caves.
Water, much like air, diffuses, so the quality of the water (chemically speaking) should not be significantly worse than the rest of the tank. The biggest problem is the accumulation of decaying material being trapped. Fish will disturb this, as will running your hand over it, or siphoning it directly during cleaning.