fish dying (long Post)

[MitchM]

So a few things going on here - I think all advice given so far is correct.
Measuring at 3:30 A.M. is probably when your PH reading is at it's lowest. For comparison purposes, try to take readings at the same time each day.
If you're only using IO without any other supplements, you don't have much buffering capacity to keep water parameters relatively stable when things decay or deaths go undetected.
Combine that with low gas exchange and that's the right combination for the deaths you're seeing, plus possibly some swings in PH, depending on the accuracy of the PH test method you're using.
Are you treating your tap water before you mix and add your new salt water?

I would suggest so far:
-Increase surface water movement
-Invest in some additional kits mag, cal, alk and test for those parameters
-Test at the same time of day each time you test
-Remove dead animals and detritus buildup as soon as possible

You might want to do a large water change (20% or so) to help get that ammonia number down to zero.