I'm still seeing spots of tissue loss on the corals. it's not like the dry regression with low PO4 but flaking of tissue, so I suspect I overdosed something or my dosages are too high. This would also have been noticeed in an increase of kH as consumption was down, but my manual testing is every 2-3 days now and I didn't notice.
Scratch that, I took some of the affected colonies out to inspect and found some large acropora eating worms - I thought I had completely eradicated these a couple months ago but it seems a few persisted. I removed about 7 and a few egg masses, hopefully this was the source of the STN.