It looks like I'm losing one of my favorite Corals. I discovered STN on a blue tort last week and it's progressively getting worse. I think it's going to be gone in a few days. As a last ditch, I'm going to frag it and see if I can save it. My fear is that I'm now seeing it on 2 other Corals and I have no clue where it's coming from.

Any ideas where this is coming from?? Params are as follows.

Temp: 77-78 per Apex
pH: 8-8.3 per Apex (fluctuates with the daylight cycles)
Alk: 9.4 dKh
Ca: 440
PO4: .03 per hanna

What I have not tested for recently is NO3. I typically only test that once a month and plan to do so tomorrow before my WC.

The following are ruled out at this point based on inspection.

AEFW - Can't see any signs
Red Bugs - Can't see any signs.