Interesting question.
Just another observation: I have a 29 gal w/ LR and some corals. No fish. I use an empereror 330 filter with filter pads. I feed corals every day, at least some brine shrimp. I have 45 lbs of LR (about 1.5 lbs/gal) and 2 in CC substrate. I also have several caulerpa strands, and get some algae buildup on glass every other day. Nitrates are 0. Why? Possibilities?:
1. the caulerpa strands take them up (and I eventually prune them, removing nitrate)
2. CC does not trap detritus which then decays slowly, rather the "trapped" stuff is eaten by pods, worms, etc.--CC can also be home to lots of critters as it is in my tank
3. filter pads, which I rarely change, do not get clogged with detritus that decays slowly, rather the detritus is eaten by pods with proliferate in my pads, the gunk you see may be aerobic bacteria
4. LR has oxygen-poor areas in the deeper parts of it, which allow for growth of nitrate-consuming bacteria
5. CC has some oxygen-poor areas, perhaps beneath LR, which allow for the nitrate-consuming bacteria to grow.
So take you pick.
*In theory*, if you have a DSB, which many say eliminates most nitrate problems, then you should get a nitrate spike during the cycle, not a slow buildup of nitrate.
kris