SPS slimming all the time

[ashr]

Hey all

Wanted to hear some thoughts on this... I have had this large stag colony for about 6 months. In the last couple of months it is constantly slimming. Some days its better and other days it's really bad. It does this at night, in the day and when feeding happens or not.

Levels:
Phos: NA - Will test tonight
Calc - 500
Alk - 9.8DKH
S/G - 1.024
Mg - 1600

Lights:
8 bulb ATI that at max is 70% for a few hours.

The coral is half way and has a power head near it. All other corals are fine other than the frags of the same coral.

Thoughts?