Many aquarists nowadays are fond of SPS corals like Acroporids and Montiporids. These tanks usually have strong protein skimmers, to ensure the water stays nutrient-poor. Low nutrient levels probably causes the zooxanthellae to decrease in numbers (or reduce their photo-absorptive pigments, like chlorophyll ), reducing brown colouration. When these tanks do not receive enough nutrient input (few fish or moderate feeding), they can become too nutrient-poor.
A tank which is poor in nitrogen (note that ammonia, nitrate and urea are not skimmed off, but that they can be kept low by food and waste skimming), might display corals which suffer from bleaching, as the zooxanthellae inside them are wasting away. They too need nutrients next to the sugars produced by carbohydrates. In much the same way, to over-usage of phosphate filters can be threatening to corals as well. Aquaria with heavy skimming which receive only moderate nutrient input could benefit from extra amino acid dosages, to prevent bleaching and improve coral colouration (corals produce beautiful pigments, in all kinds of colours. This is really a sunblock which allows corals to regulate the amount of light they receive, and to produce it corals need amino acids or proteins).
Scientists also found that amino acid uptake is stimulated by light, by using metal halide lamps at different heights. Above 160 micromole photons / second / cm2 there was no extra amino acid uptake. For aquarists it means they don't need to put 400W bulbs above their (average sized) tanks to ensure amino acid uptake. However, it is interesting that amino acid uptake is a diurnal event.