sps bleaching vs dying

[likemike99]

So my tank is 75gal 2.5 years old. nothing new has been added in 6 months. all of the sudden a few stags and 1 acro started turning white on the bottom.(shaddowed area). None of them are really big pieces that should be blocking too much light. parameters all stable

Salinity - 1.026
ph- 8.0
alk-10
nitrate-10 I know , but i like to allow it al little higher- I get better growth with more food around (food=bacteria to eat the nitrates- not the actual nitrates) - it does not fluctuate either
phos-0
ca-400
mg-1300

I do not see any red bugs or anything during the day or at night.
any ideas?
I will try to post pics tomorrow

 

[Reef264]

Temp Swing?Stung by another Colony?

 

[karsseboom]

Have you checked for aefw? They can be hard to see

 

[likemike99]

in the pic you can see they are not too close to any other corals. I don' think the temp is doing too much, but ill keep an eye on it. I have not been able to see anything on the coralseven with magnification. the 2 corals are at the top right of the tank pic. you can also see it going on in the actos in the bottom