they do.. but there is a balance, roughly 16:1 NO3 to PO4. If your nitrates are high and phosphates are undetectable it can lead to a bacterial imbalance, which can cause cyanobacteria, dinoflagellates, etc. Also, corals use both nitrate and phosphate for building the tissue/skeletons. If they are out of balance it can slow/impede growth as well as cause "pastel" colors - think ultra low nutrient zeovit type coloration. Look up the Redfield ratio, it should help to describe this a bit more. I dont shoot for matching 16:1 but I do try to keep my nitrates around 7 ppm and phosphates around .15-.18 ppm.. I find if my numbers drop too much below this all my red and yellow sps turn to pink and green pretty quickly.