SPS help

[delo978]

Need some help/suggestions for my SPS....never ran into this problem before and did everything I could think of. I upgraded from a 50gallon tank to a 90/130with sump tank and everything was perfect for the first 2 months of the transfer. I hadn't changed my routine and upped my dosing to a my new tank size all the same chemicals I was using prior. Starting a few days ago I lost 1 SPS coral (acro) within 24 hrs and I've lost 3 more in the last week. There was no symptoms they were fully open all day everything peachy and Just out of no wheres they turned white within a few hours. My parameters check and are as follows (as some may ask what exactly they are)
PH- 8.3
Salinity 1.024
Ammonia- 0
Nitrite -0
Nitrate -0
Phosphate - 0
Copper -0 tested just in case
DKH -11
Calcium -400
This is what I've been dosing for the last 1.5 years in my previous tank and have kept dosing since the upgrade.
Calcium, strontium/magnesium, iron, potassium, DKH buffer, and lugols....I might be missing one but not currently home to check. Here's what I've done so far to attempt to rectify the problem....water change, change the carbon, clean out all the filter pads in my return pump, cleaned out the entire skimmer, cropped out some cheato. I can't think of anything else I could do. all my softies and leathers are fine and the sps that are alive are all open with no sign of stress. The only measure I can think is to stop dosing everything except calcium and see if it's maybe it's a balancing issue since I don't have a test kit for stron/mag and hope that it sorts itself out. I've also checked for parasite like redworms and nudi branches however haven't seen anything. Any suggestions would be greatly appreciated.

If you're inquiring about my tank setup It's a standard 90g drilled tank to a 30gallon sump with cheato night light cycle. I've got a wavemaker2 and a coralia 125 skimmer. lighting is 2 400watt MH 20k bulbs and my temp doesn't fluctuate more than 1deg by night. Night lighting is a 20 LED strip for visual effects only it's not bright enough for corals to stay open at night. light timing is 11 on 13 off. Not really sure what other info you would need but ask away and I'll provide what I can.

Cheers
and thanks for your help.