In my opinion it is the nitrate phosphate ratio too. If you are running a tank for a long time corals might adapt to a high nitrat phosphate ratio but the growth of corals is better at a nitrate phosphate ratio below 10 and with phosphates at .05 to .1.
When there are some good tanks with low phosphate concentrations and a high nitrate phosphate ratio usually they run it with a CO2 coral rubble reactor. This device produces a continuous stream of phosphate and usually it is hard work to keep phosphates at .03 with GFO. In any case there is enough phosphate in the tank for corals to grow.
I think there is a fundamental misconception of nitrate and phosphates. To me it frequently sounds as if nitrate is the good one and phosphate is the bad guy. In fact it is just vice versa. Phosphate makes the corals grow while nitrate makes them brown. Tissue necrosis, either basal tissue necrosis or burnt tips, occur at low phosphates while usually nothing happens when nitrate is below the detection limit of most test kits. With low nitrogen the corals get lighter and maybe pale but usually tissue stays intact and healthy. In fact hard corals can make their own nitrogen compounds with the aid of mutualistic nitrogen-fixing bacteria.
