SPS White bottoms

[Chip]

I have a few corals that are turning white from the bottom up. Anyone know what is going on.
Alk. 8.4 Cal.440 Mag.1400 On a Calcium Reactor with ARMs media. With 3 Radion Pro Gen. 3 My 300 gallons has been up for 2 years.
Thanks

 

[Chip]

 

[chefjpaul]

NO3 & PO4. #'s

Or
You have a mermaid eating them

 

[Chip]

NO3 & PO4. #'s

Or
You have a mermaid eating them

both are reading 0. I have the skimmer off and have not done a water change in a mo. I cant get this tank dirty.

 

[twilliard]

Here is the thing about zooxanthellae algae.
They need food in the form of no3 and slight po4