Bleaching favia and tissue necrosis on millepora

[Aquattro]

First, bumping your salinity to natural levels will help the other parameters, 1.026.
Anyway, ya, a sharp and quick drop in PO4 can annoy things, but I wouldn't expect tissue loss. Not impossible, but wouldn't be my fist guess.

[Myka]

How are you testing phosphate? How much time between it going from 0 to 0.5 ppm, and how long was it at 0.5 ppm?

[SmallFry]

Salinity is possibly a little low, if I'm honest, I bought the hydrometer when I started with saltwater a few years ago and it had a green band on it so I set the S.G. plumb in the middle. Thing is I've kept doing it that way because I haven't had any problems up to now (how many times have we heard that before?).

Phosphate was measured with an admittedly quite old API kit. The drop in phosphate would've occurred in less than 18 hrs. No idea what it usually runs at as I haven't tested for anything other than Ca Mg and Alk in ages. This is the first time I've ever detected any phosphate with a test.
Until recently I had so much algae that nitrate and phosphate always registered as zero. I guess I've gotten rid of enough algae now that it actually shows up.

This is kind of the issue I'm having - I've arrived at a point where I have lots of variables and very little in terms of a baseline and nobody to blame but myself..