Deadly Spike in SG

[RHagemann]

I have a 72 Gl reef W/ Overflow, Sump (divided into thirds....drain is split 75% of drain goes into the skimmer side (left), the remaining 25% goes into the fuge side (right), both overflow into a center return section. In the fuge side, I have a 4" sand bed, approx. 15 lbs of LR rubble, and a medium sized ball of chaeto.

I tested water last week

Nitrates - 0-2
Nitrites - 0
Ammonia - 0
PH - 7.9
SG 1.023 (a little high for my tank...it's usually .20 to .21
Temp 80.3

I left for the long weekend and returned to find my BTA's melted to nothing, Pulsing Xenia heads all melted, Torch heads melted, zoanthids all closed, Blasto heads all melted, Frogspawn withdrawn, button polyps closed....you get the picture. All fish look healthy (Purple Tang, Flasher Wrasse, 2 Percula's (searching for their BTA), White Sleeper Goby.

I did a 20% water change immediately, as it was due anyway. The only thing that had changed was my skimmer (pump went out so I had my reefsitter unplug it).

The water has a foul smell (assuming it's due to the die off of the corals?). I started to run tests this morning and my SG is off the charts! Like....1.040 and that's AFTER a major water change yesterday.

Any idea what would cause such a spike in SG? I had pre-mixed water for my reefsitter to top off with, and it was mixed to 1.020.

Any help would be greatly appreciated. I'm off to go get more water....would say...a 50% water change of very low salinity be advised?
As much as 1.040 scares the life out of me, dropping it to the .20 - .22 range in one fell swoop could be just as deadly couldn't it?

V/R,
Rob