[Uncle99]

I assume your PH is low as well as Alk say 7.6-7.8
Watch chasing numbers, they can be deceiving, your number are a bit low, but not of great concern.
Since you have corrected the water, have any other fish died?
Do you have good surface agitation through some type of powerhead/wavemaker?

My PH I can't get above 7.8-8.0 and my Alk I can't get above 8dkh. This rebuild 15 months old now and I have not lost. A fish, a coral, not even a snail.

Let's just get the water stable and consistent, normal 1 week 10% change likely good, but new fish continue to die, this sounds more like a disease.

So my parameters are:
78.5 degrees
1.025 salinity
7.8-8.0 ph
8 dkh Alk
420 calcium
1360 mag
Nitrate 5ppm
Phosphate 0.12
Ammonia 0
If all your tests are close to the above, your water is on point to NSW, more or less.
If anything dies with the above parameters, more or less, then the problem of water is ruled out.