[bertoni]

I ran a tank with a coarse substrate (crushed coral) for a while. The crushed coral seemed to trap organic debris, which then decayed and released mineralized nutrients, phosphate, and nitrate, into the water column. Replacing the substrate with very fine sand fixed that problem. Even a coarser grade of sand can trap a lot of debris, and I suspect a sufficient organic load can contaminate even very fine sand. What happens in any specific tank will tank some examination and thought to understand. I don't think there are any guarantees that you can make without looking at the substrate in question.