Whats wrong with my coral ??

[FWC]

I did a water change roughly 1.5 weeks ago and all my SPS decided to start crapping out on me. Everything was colorful/colring up ,and I have awesome polyp extension ,then after I did my usualy water change the same as always ,the corals become un happy.

They are all browning out and not showing near as much extension as before. I don`t know what I did wrong ??

Nitrates=0
Nitrites=0
Phosphate=0
pH=8.0
ammonia=0.6

The ammonia has always been around 0.6 ,so I dont see why all my SPS would suddenly start to go down hill.

Also I'm running 2 T5-0H's at the moment ,and the same 12 hour light cycle since I started the tank.