[Mark]

Yes, I certainly think both depletion and accumulation of various elements contributes to the old tank syndrome phenominom.

Crowding large amounts of corals creates a lot of competition for resources, which the aquarist must account for by replenishing. But I worry that the rate of depletion can get excessive, and perhaps that the aquarist is not replenishing all those needed elements. In the real world, overpopulation leads to the limiting of resources. I think the same applies to our tanks. On the same note, corals compete with another for limiting resources(like real estate and food) by trying to eliminate their neighbors. The byproducts produced by corals to compete against neighboring corals could also build up in the reef aquarium and negatively affect the organisms in the tank. Water changes, carbon, and skimming may help a little. But I think it is inevitable for such byproducts to build to unnatural levels in a closed system. Captive corals have to put up with a lot of stressors, and I think this is yet another kind of stressor.

I haven't looked into RTN much, but I wonder if RTN is less prevalent in tanks with lower sps densities. It'd be interesting to find out.