okay so i have been reading threads on how they have made rock for there marine aquariums. Now i wanna try it! so let me show you how i think i can do it can you guys tell me what i need to change? So first of all what cement mix should i use? after i figure that out id get a 50g bucket fill it up with the mix and add water and sand to it. But i dont uderstand how the sand creates holes in the concreate for the bacteria to live in. After i made the cement block i would rince it thurley leave it out side for a week in the sun and then get a bucket and rince it in a buket with prime. Then add it to the tank. So what do i need to change in my game plan? thanks guys. (by the way for maximum bio filteration would 700pounds of rock be enough?)