I have several observations on this:
First, the pH is not low enough to indicate a condition of extreme O2 depletion, CO2 build up.
Second, the only way to crash the oxygen saturation is to stop running the over flow into a sump or part of the sump or a makeshift sump while working on getting a new refugium. Did you do that? A good over flow and spillway like it should be done, should be vortexing the water enough to compensate for a lot of the CO2 build up at night. Since CO2 is heavier, it can also go over the overflow and to the floor, thereby creating a clear zone for surface agitation at night in the main tank.
Next, what is the measure of the ALKALINITY? Meq/L or dKh ??? With that calcium, dkh, pH and Mg, it looks like you have a major shift in the buffer system to a secondary Mg buffer and that you are very out of balance. The corals care less about all this than you can imagine, but, its very hard on the kindneys of the fish. In an extreme, Uremic blood poisoning is then quite possible in the fish. I'd have to see the tank, but, its really not that easy to keep things in balance once you start adding kalk and buffers.
Next, when you fragged that Kenya tree, it was done inside the tank. You need to be running Activated Carbon to remove that toxin created there as well as to remove the Chelators in the I/O (or almost every other brand too) new salt mix. Since you do not have a deep sand bed, mass macro algae beds or deep rock substrate in there, O2 depletion is not as likely to happen at night from microbiol activity. It may go down, but, depletion is another thing.
I am assuming that you are still running an over flow into something. If not, with all that coral in there feeding at night, oh man, you gotta have an overflow going to a sump.
Don't go wild with water changes since it looks like the buffer system is messed up. A little at a time, a couple times a day to equal not more than 10% per day! Do 10% per day for 10 days and then see where you are. The corals may go shocky on you. Run the carbon and don't sweat it. They'll close up till all is stable again.
Hope something here is helpful.