The background is that algal growth in reefs is limited by nitrogen (nitrate) and iron. Corals need phosphate for growth and they can't extract it from the rocks and gravel like algae and cyanobacteria do. So I give the corals enough phosphate and trace elements for good growth to outcompete the algae in nitrogen (which simply is not possible with high nitrate ratios or concentrations). Until the low levels of nitrogen (and iron) are achieved the corals may darken, but as soon as the zooxanthellae get nitrogen and iron limited they lighten up and colors of corals improve a lot.
The pastel colors achieved in my way of keeping corals have nothing to do with dangerously low nutrient levels or a critical situation. Nitrogen limitation affects zooxanthellae (algal) pigmentation, that's all. In my experience it is not possible to kill or even damage a coral with nitrogen limitation as long as you have a few fish that get fed in the tank. With phosphate the situation is completely different.
In fact it is vice versa, nitrogen limitation limits zooxanthellae growth (like algal growth in general in reefs) and initiates the energy transfer from zooxanthellae to the other partners of the coral holobiont. Corals may even regulate the behavior of their zooxanthellae by limiting the nitrogen transfer to the zooxanthellae.
In my eyes this is the way that is most stable and makes the least problems, but again experiences may differ.
