Whats going on with this monti?

[jimrawr]

I've had this piece for like 2-3 months and it always looks bright red and healthy. Coral has been growing fine. As of two days ago, it started to get all white and have holes where the polyps used to be. The picture looks off on the color. The area where you can see tissue is really a nice pretty red color still.

Parameters are stable
SG 1.025
CA 420
ALK 9.3 dkh
MG 1350
Temp :78-80
NO3 = 20
PO4 = .05

The only thing I can think of the changes recently is that I did a 3 day lights out due to Cyano, but nothing else was effected and its not the first time I did this and coral was always fine.

Thoughts on what could be going on? Everything else in the tank seems to be fine although I do have one acro that looks to be STN'ing but its a fairly new piece that never took hold or grew at all.

 

[Bill Nye]

How long have nitrates been at that level? Seems pretty high.

 

[jimrawr]

For a decent while.. they seem pretty steady right around 20ppm

 

[mcfa2403]

I would suspect that the issue is the alkalinity, my mystic sunset montipora (looks like yours may be the same species) tends to do that if the alkalinity is maintained above 9 for too long which I hate because above 9 is where I get the fastest euphyllia growth. The weird thing is it seems to take a while before it happens but then it will happen quickly but immediately recovers when I drop the dKH to around 8-8.5

 

[jimrawr]

Ya I'm going to try to see if all around 8.5dkh helps at all

 

[jimrawr]

I just noticed the water is a bit smelly when I went to do something in the sump. I added 1 cup of carbon today, I wonder if that could be an issue?