So there is no rock. No need because rock isn't a filter in Coldwater. Foam is pretty porous but biofiltration in Coldwater only compliments. You have to focus on mechanical and chemical filtration. So I'm the pic you can see that there are two chambers. One is the overflow where the mechanical and chemical(chemipure elite) filtration will be. That flows into the return chamber and chiller is. Please don't mind the mess