Bleaching is a fairly complicated phenomenon, and I thought it was more related to heat, lighting, and perhaps disease than phosphate and nitrate.
Phosphate begins to interfere with calcification at about 0.03 ppm:
http://www.advancedaquarist.com/issues/apr2002/chem.htm
Both phosphate and nitrate can cause coloration changes and perhaps encourage polyp bailout and other problems, as well.
Alkalinity should be maintained between 7 and 11 dKH:
http://www.reefkeeping.com/issues/2004-05/rhf/index.htm
Low alkalinity will prevent calcification and limit growth.