I have been fighting the same issue on 2 tanks. One with biopellets and the other without. I removed all GFO, carbon, and the biopellets. I also increased fish feeding and coral feeding at night. The tank which had the biopellets has stopped STN on a tricolor and it is growing back. I have a few other acros which show very slow STN and PE is very little to nonexistent. Then I also have other acros which are growing as if nothing is wrong with excellent PE. Birdsnest, stylophora, and montis are normal.
The tank which never had biopellets has a number of acros with STN while some acros have great color growth and PE.
The common largest change before all this happened was that I increased lighting on both tanks. Shortly after I had remendous growth spurts in both tanks but N03 and P04 dropped to 0 about 1 month later.
I suspect the growth spurt caused the corals to use up all available nutrients and the chemical filtration compounded the problem. I have since fed the tank even more and reduced lighting. N03 is still low .01-.02 but detectable and P04 .02-.03
Both tanks seem to be bouncing back.
Tank which had biopellets had a 15k 250w mh swapped to 10K and LEDs at 80% added for supplementation. Other tank is all LED and I was running 80% blues and 20% whites.
15k mh is back in now and LEDs down to 30%. All LED tank had some LEDs removed and both channels down to 30%.