Bleaching favia and tissue necrosis on millepora

[SmallFry]

Hoping someone can help, I'm a little lost..

I'll start right from the beginning.

Long ago I had big algae problems, have been running Purigen, Rowaphos and intro guard bio cubes for about 9 months or more. Bio cubes seemed to do nothing, so added Purigen/Rowaphos a month or so after the bio cubes. Algae slowly reduced. In the last few months I got lazy about changing the Rowaphos and Purigen.
You'd think the algae would make a comeback, but it actually reduced a lot - has been sloughing off my overflow to the point it's now almost spotless. Still get some growing on the tank glass though. I concluded the Rowaphos/Purigen was stripping out something the carbon source needed to operate.

About a week ago I noticed my Favia was loosing colour on the brown outer edges of the polyps, I thought maybe it was too much light, so I put white paper on the between it and the light which made no difference. I then thought perhaps the nutrients had become too low, pulled the bio cubes.
Fed mysis with liquid to try and boost nutrients (always coincided with rampant algal growth in the past).

Tested after day or so and found nitrate 0, phosphate 0.5ppm, thought perhaps it was too much phosphate or the fact I've not been running carbon for a while. Added fresh Rowaphos and carbon.

Today I see that I have some sort of necrosis on the base of my two Millies - not surprising its affecting both because they are from the same original frag.

Currently:
Alk 6.5dkH am raising it aiming for 7.5
Ca 370ppm
Nitrate 0ppm
Phosphate 0ppm (0.5 last night)
pH 7.8
Temp 27
S.G. 1.023 (this is from a hydrometer, so the absloute value may not be quite right, but it is consistent with what the tank has always run at)


Right now I'm thinking that the 0.5 drop in phosphate overnight is most likely for the necrosis on the Millies - I'd be interested to hear if that's likely. Also any suggestions to remedy (more mysis liquid?)..

Also any theories on the bleaching of the Favia would be welcome.

No new additions for at least a month or so...


Thanks in advance,
Rob.

Edit:
I have also taken GFO and bio cubes offline.

[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..