bugs ore not

[Zoaelite]

I had something similar happen to all my SPS and I found it to be a combo of CA/ALK swings paired with my biopellets. Doug (Fishytime) did a really good job of explaining it, can't quite remember but I removed my pellets and have never looked back. I know your not running pellets but CA/ALK swings leading to pH swings can wreck some major havoc.

The problem with STN is once it starts and the coral is immuno-compromised it's a very slippery slope towards death. I would personally frag all the dying coral and frag epoxy your tank with some new frags.