For over a month now the bases of some of my SPS corals have been receding. Check out this picture:
I'm kind of at a loss as to what might be causing this. Water parameters seem fairly normal to me: Alk 12.8 dKH, Ca 440, SG 1.026, Temp 79, NO3 ~3. The tank is 240 gallons (8x2x2') with a...