William,
by any chance, do you have any single type of soft corals or polyps that are in a lot of places, like GSP or red/green mushrooms, or blue clove polyps? Anything that has spread around a lot in your tank. I'm not referring to physical contact between the spreader and your acroporas, but have wondered if this type problem could be related to chemical secretions from the prolific spreader.
I have been having the same issues as you for several months, I just haven't posted about it. No pests, all chemical parameters are good. I have tried changing literally everything. New bulbs, GFO/No GFO, took the nitrate reactor off line/put it back on line, GAC/No GAC. Had Joe double check my chemistry levels with his kits. Did a 25 gallon WC every day for two weeks straight with D&D salt instead of my usual IO. You name it and I think I have tried it.
There is a thread of Reef Central about the same thing, and some folks are sure it is a bacteria issue or that the live rock in a system with this issue is clogged up and needs replaced. I'm not going to do that.
The last thing I am trying is to eliminate all the blue clove polyps from my system. Over the last year they have spread to cover about 85% of all the light receiving bare live rock in my 210 SPS tank. They are not aggressive, in that they are easy for acroporas, even Montis, to kill. But they can crowd out zoas and palys.
I am wondering if the sheer mass of them may have something to do with my STN issues? I have literally 350 lbs of LR and they looked like a blue carpet over 85% of it. Pretty, but I wonder if they are overdosing the system via allelopathy. I run GAC, but not a ton of it (one single BRS media reactor).
I have been using my reef napalm to kill them over the last couple weeks, and I only have some stragglers to take care of now. If this wasn't it, then I am totally out of options and am just going to let it settle on it's own and no new acros go in the tank until it settles out.
Dave