Need a little help! STN or shadow die?

[Oscar Bravo]

I had thai piece for a year and half and just this weekend showed some STN. I don’t know what it is, parameters are on target. Can it be lack of light there. I also thaogut about parasites but I see none. It looks like something took a bite out of it. It started small now is bigger in two days (see pic). My fish are not interested on the corals. I don’t want to frag, is my only small colony

 

[Oscar Bravo]

Here are pictures at night.

 

[mrtian97]

I think is STN, I have similar frag that happened during my vacation (doser went out of whack), but after stabilized, the frag grew the skin back, I think urs will too

 

[Oscar Bravo]

My Cal 410 Alk 8.4 Mg 1400 and ph 8 from 7.7 over days. It has never change. I did change the method of adding Kalk, 5 Gallon into my ATO. Parameters didn't not change only my ph and that was my target. So now I am back to just RO water on my ATO starting today see that works.

 

[VTBig053]

Any changes to your flow recently?

 

[Oscar Bravo]

Hmm as a matter of fact, yes. I return my gyre for repairs. However I increase my other gyre to %80 percent to compensate. Or maybe %80 is too much, polyps are swinging back and forward.

 

[29bonsaireef]

Looks more like a patch of RTN, STN is slow to spread and not so much of a stark white spot. IME, Zoox are weaker and more susceptible to any changes on shaded parts of your corals. If anything goes out of whack usually the undersides of corals are the first to be effected, that is only my theory though. Doesn't look too bad, and if you're able to keep things stable it should be okay. I would check close for pests just to be sure though. Always nice to catch them early if you have them.

 

[Oscar Bravo]

I am keeping an eye for any pest at night. So far none.