This week, everything has suddenly been dying in my tank. My corals were great up to this point and now none have opened in days and are starting to die. Both my sand sifting stars died, my brittle star died, 4 large snails died, and my tuxedo urchin died. Not sure what is happening. I tested my water and everything checks out. Phosphate - 0.1 ppm Nitrate - 5 ppm Ammonia - 0 ppm PH - 8.1 Calcium (DKH) - 9.2 Salinity - 35.6 ppt. Please help