ALL coral need NO3, every last one of them. But in excess it will cause a brownout in some corals due to an increase in the green red zooxanthallae that are more able to utilize the nitrate... Algae that are harmful to coral? Bryposis, Derbesia, and Valonia can all overtake corals. Dinoflagellates (not algae but are increased with NO3) produce toxins that can kill sensitive invertebrates, usually snails and crabs. A small level isn't bad, but better growth and coloration have been observed in many corals in immeasurable NO3 tanks. Please read Nitrate in the reef aquarium by Randy Holmes-Farley, there is a wealth of information there. Higher NO3 also lowers ORP. Guru you said that you dose sugar, I believe that will have an effect on your NO3 levels... Could be skewing things... But I agree... People see their tank reading 10-20ppm of NO3 and freak out. There isn't really any reason to do so, unless they just did an 80% water change, and THEN tested 10-20 ppm NO3.