Danny,
It may not be that NO3 is totally unbeneficial for corals, just that above a very low level it becomes harmful rather than helpful. In other words, the corals may use NO3 at low concentrations, but since the oceans are usually NO3 free since different organisms like macro algae & phytoplankton use it up, corals are not adapted to thrive in high NO3 environments. My softies, for instance, seem to do better when the water is not completely nutrient-free.
Anthony
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