I noticed three areas in reef today with white skeleton showing, which seemingly came out of the blue. Not sure why so I did a full set of tests to see what may have changed. Nothing major cropped up but these are my findings. Specifically the base of the Staghorn colony has been affected, as has a Montipora colony and a birdsnest colony. STN is slow tissue necrosis, a creeping death that often happens from the base of the colony upwards.  Here are a few visuals. 

Here are the water test results: https://melevsreef.com/blogs/water-tests-and-what-they-mean-me