The type of rock makes all the difference. You want a nice light, porous rock. Heavy, dense rock, whether it's live or dry is useless, and will just lead to frustration with your nutrient levels.

The more porous the live rock the better, this means more surface area for benefitial bacteria to thrive

That is not 100% true. Rock that is too porous can be also problematic. Dense rock has the anoxic areas that harbor the bacteria that convert no3 into nitrogen gas.

I mean the structure of the rock itself, not that it necessarily has more passageways etc... though this is important as well IMO. Water needs to be able to penetrate the rock in order for it to function well.

That has little to nothing to do with what makes live rock function.

Interesting, please explain.

http://www.fishtanksdirect.com/Produ...anch-large.jpg
Tonga Branch


http://i636.photobucket.com/albums/u...k/IMG_0610.jpg
Porous Rock

Tonga branch as far as I know is some of the densest LR available on the market. My one tank had a majority of Tonga branch. Everyone said the tank looks like it did not have 1 pound per gallon, but some of the larger pieces were around 40Lbs! A 40 pound Fiji LR would be 3-4X the size.
Most tanks do not need more surface area for the bacteria that convert NH3 or no2. They process it quite well, and you do not even need LR for this. Bio balls even work for that process. There is then just a build up of NO3 left. The only thing that can remove NO3 I know of from a reef aqaurium is anoxic zones in LR. However if the rock is too porous and there is too much water flow through the rock these zones cannot establish due to the higher oxygen levels.

Personally I have never found the porosity of LR to effect the biological filtration. However I could see porous rock acting as a much better mechanical filter.

So you're saying the de-nitrification is taking place on the surface of the rock? I've never heard of this, I always thought the anoxic zones where inside the rock. Can you point us to references for this, you've got me very curious.

Uhh I am sure I can dig it up. But I think somewhere the point was lost.

The area 1-4" beneath the surface has enough oxygen to harbor the bacteria that convert ammonia and nitrite. Rock that is dense enough or porous and large will have an anoxic zone in the center where nitrates are converted into nitrogen gas.

And yes if you have sufficient surface area in your tank covered in bacteria, that will act as a bio filter. That is the whole point of bio balls.

I suspect the anoxic zone begins much closer to the surface. IMO the bacteria living in the top layer of the rock would use up the oxygen very quickly. I've always had 0 nitrates with my porous, thin, loosely spaced rock, and I like tons of flow.

I'm not sure if any studies have been done on the distance oxygen penetrate live rock, but it would be interesting to find out. Where did you get 1 to 4 inches from?