[Electrobes]

As a guy who makes rocks for saltwater tanks as a hobby, my biggest problem with the sponge idea is strength in general.

It takes a good while for bacterial/etc to penetrate the thickness of any rock, we're talking inch+ per year... it ain't fast.

My biggest fear is the core of the rocks being fairly weak. Between people stacking it, drilling it, etc... one fracture will most likely destroy the rock. One would have to be fairly careful not to have it be physically hit too hard or tinkered with (Drilled, chipped, etc) so as not to destroy it. You would most likely have to keep the rock on top.. not under the other rocks.

Even if you tried to thicken it enough to it could withstand the physical realities of us and our tanks, at what point does it get too thick for the sponge (Assuming the idea worked) to work?

Overall, while I love innovation, I don't see this working out all that well, IMO.