Your corals will probably be happier with a lower nitrate level. Try to get it down to 5ppm or less. Regular water changes to keep the level down, along with a good salt mix, should also keep you alkalinity and calcium levels up. 425-450ppm of calcium and 8-10 dKH of alkalinity is what's normally recommended for hard corals (your Duncan), so alkalinity is equally as important, and some will argue that it's more important. It takes proper amounts of alkalinity (carbonate/bi-carbonate) and calcium for corals to build their skeletons, as they combine the two to form calcium carbonate.