I've gone (going?) through the same STN regime. Tanks @ 8x months now. I kept targeting 1 thing at a time trying to figure it out. On top of that, my tank is sumpless.
I think the most impactful discoveries I've made so far were:
• Don't done anything directly into the tank. Everytime I did this, the next day I had some type of tissue recession. Anytime I've dosed it into my HOB filter, no issues next day
• Quality RODI. Found out my city uses chloromine. I always had 0-2ppm (when I catch it at 4 I would change DI), but was using a tiny RO buddie. Can't say 100%, but im pretttttty certain chloro got past it, especially 6x months into use. TDS alone couldn't tell me for certain
• Recently changed it out for a
BRS 5 Stage, added a chloroplus stage before the carbon, and made the DI dual DI. Might be overkill, but now @ 7stages, I can say for certain if quality water did help. Doing water changes to swap out all the previous water for this pristine stuff
I also started with Dry rock, and one small mushroom live rock I got from a fellow reefer, so maybe 40lb dry, 3lb live. Could the dry rock be the cause of all my issues from the beginning? Not sure!
I have had coral encrust on it, acros & montis, so I want to say it is most likely not the dry rock, but of course it'll be too hard to tell!
Most acros that had STN are healing quite good. Maybe 50%+ recovered over the exposed surfaces. The ones struggling (but haven't gotten worse) are the ones covered with algae on exposed skeleton.