Coral dying, Salinity spike!?

[Oggey]

Have a 75 Gallon mixed reef tank with sump, skimmer, cheato, 2 part dosing. System been running for 3+ years

My wife noticed a bunch of empty snail shells at bottom, more then few extra I have for hermit crabs to use.
Looking deeper found bunch of snails had died, blastomusto pulled in and not opening at feeding time, the green slimer has black/purple tips, hollywood stunner and montipora turning white and branching SPS turning black/purple
Fish good, CBS fine, sea fans fully extended but soft coral only partially extended
Numbers good, Alk 11.7, Calc 420 and phos 0 but found salinity had gotten up to 1.031.
Recal'd salinity checker and over next 3-4 days brought numbers down to 1.025-26.


Been a couple weeks
Snails that didn't die are out and about, hadn't seen many on glass during initial issue
Coral still dying, 3/4 of stunner now white and most of SPS branches black
Just noticed cheato in sump turned white and died sometime in last week
Salinity stable at 1.025 and Alk/Calc/Phos still good

The Hollywood stunner was as big as a saucer and green slimer and other branching SPS filled top of tank, now all turning black and not extending polyps
Tried taking pictures but crappy phone and blue lights nothing came out right

Is there something else I'm missing, will coral recover or should I pull out, or maybe cut off dead parts/pieces?

While I was typing this one of the snails crawled out of tank and heard him fall on floor!? So bad their crawling out of tank!?

Thanks

 

[T-J]

Check for stray voltage?
Nitrates?

 

[homer1475]

If your PO4 is truly 0, your starving your coral.

What test kit are you using to measure phosphates?

 

[Oggey]

Check for stray voltage?
Nitrates?

Have ground wires in tank and sump because problem couple years back, still working
Just tested Nitrates, reading over 10, using red sea kit and looks like color in between 10 and 20.
Could this be from the snails dying?

 

[Oggey]

If your PO4 is truly 0, your starving your coral.

What test kit are you using to measure phosphates?

Red Sea Marine Kit

 

[homer1475]

I would get a measurable amount of phosphate in the water. The damage is already done, you may very well end up loosing those pieces.

Can you post pics of the corals in question, and a full tank shot?

 

[Oggey]

Photos