[Sk8r]

Depends on the rock. We have one rock in our freshwater that probably weighs 40 lbs on its own. it would be horrid for saltwater. Some of my dead coral has a negligible weight, light as a feather unless wet. The main thing is surface area---and porosity [micro-holes]. That is a FAR more reliable guide re the efficacy of the rock. The same way that human lungs, if truly flattened, would cover a football field, your rock needs to provide a lot of porosity.

Somebody is bound to ask about the artificial expanded aluminum/ceramic blocks. I have one of these: I have run a PolyFilter strip to test for free aluminum and it is safe. It is also porous as a sponge. I would, because you need the sort of bacteria that lives deep in the dark, go for a block of this stuff rather than the small pieces. OTOH, it would not look very good in your typical tank, and while a sump can get flow through it, the typical in-tank situation, not so much. I also did not experience a nitrate reduction by using it (and I had a problem). I got the nitrate way down real fast, after using everything from ceramic rock to carbon dosing and several absorbent mediums---by getting a much better skimmer. Nitrate sank like a stone. Rock---can only do so much. Sometimes it needs help.