Thank you! I just set up an air pump on my tank that I have bubbling in the rear chamber. (Pumping in outside air) i know my probe is a little off but its not far off. It has already risen quite substantially in the 1.5 hours its been on. I also ordered a mini skimmer to hopefully add some more oxygen. Air pump will do for now. I'm still going to QT just as a precaution.So - as you've suspected, in most cases where all the fish die, but the corals are fine, the issue is some disease with the fish. Given the lack of prior symptoms and rapid death, Amyloodinium (velvet) is one possibility.
Water quality issues can be ruled out with just one exception: oxygen/carbon dioxide levels. If those are out of whack, the fish can die overnight, but the corals and most invertebrates will be fine. Then, things get really weird because the gas exchange is affected by things like how full you fill the tank. I've had cases where the powerheads or HOB filters made enough bubbles to create good gas exchange - until one day, you fill the tank up an inch higher, the bubbles stop and the fish die that night.
I'm looking at you pH of 7.4 - either your pH probe is mis calibrated, or you have too much carbon dioxide in the water. That points to poor gas exchange. If you aerate the water well, the pH may still be a bit low in t he mornings, but should rise to 7.8 or higher at night.
Jay