So *maybe* I've figured out what's been causing so much trouble in my tank. I'd been going through everything, checking temp with a reliable analog thermometer, cleaning pH probe and checking against another pH meter, checking my RO water.... recalibrating my refractometer. I found that the refractometer was off a little, making it read low. So after adjusting it I brought the tank salinity down from maybe 37ppt to 35ppt. Didn't think that was enough to cause so much decline though. I sent off an ATI ICP-OES test sample last week, so we'll see what that says.
Then I thought, well, my calibration fluid is pretty old. Best not trust it. I ordered a single use Hanna 35ppt packet along with pH probe calibration fluid. Checking the refractometer with the Hanna fluid I found it reading 30ppt. Yikes. After calibration I checked the tank and found it was at 40ppt! And so was my old calibration fluid of course. (pH probe calibration was fine.)
So I'm thinking that the high salinity may have been causing die off which then was causing increased nitrate and phosphate.
I did a 30G water change with 30ppt water today and will do two more to get back down to 35ppt. Changing 90G in 3-4 days should help with water quality a bit too.
I'm hoping anyway.