Need help with sps issue.

[Tomb85]

This is my first sps coral to see if I can keep them. I have a green digitarta but under the polyps on the coral skeleton there are brown areas. I'm not sure if its algea or rtn/ stn issue.
Parameters are as follows,
Salinity 1.025
Ph 8.13
Alk 8
Phosphate 0.3ppm
Nitrate 15ppm
Calcium 430
Need to buy magnesium test kit, keep forgetting.

 

[Tomb85]

This is the coral

 

[Tomb85]

Also running 2 maxspect gyres at 50% flow to create good water turbulence. Maxspect r420r light on a 10hr cycle. Filtration is fluval fx6 canister cleaned monthly. Also running maxspect qq3 skimmer on 12hr cycle when lights are off. If I run it full time I lose to many nutrients out of the water which stunts the coral growth on my other corals. Tank is 17 months old now.

 

[thaistalyn]

It's a little bit hard to see with all the blue. I'm guessing you are using a phone, try to switch it to manual mode and turn up white balance to 10k or just turn on some more whites on your lighting, just to get a better pick of the sections you are concerned about.

Saying all that, I do not see evidence of RTN/STN as those lighter colored areas look to have polyps and no tissue loss. In general, the coral seems to be pretty healthy.

In my experience corals tend to "brown out" with too much nutrients (ie NO3 or PO4) or too low light since the zooxanthellae are algae and they are getting fed well or not getting enough light! Your PO4 at .3ppm could be considered high by some people, but if the coral is having polyp extension, mostly good color, and growing, stick with keeping your parameters stable and accept that .3ppm PO4 works for you. If you want to see if the standard answer ".03 -.05" PPM PO4 is best for SPS, then try some phosphate reducing techniques such as GFO, Phosban, Phosorb, ROWAPhos, Lanthanum Chloride, etc... to get your PO4 lowered to the optimal zone.

My advice would be to find the parameters that keep your corals happy and colorful and keep the tank stable at those parameters!