I don't think the ph is playing any role in your troubles. My phis at a steady 7.8 and all is well in my tank.
I was running my 125g tank with 250lbs of rock and still managed to do a large water change. I would for sure increase the amount of your water change by a bit, don't worry about the water level.
A water change is a great thing and can fix many issues believe it or not.
Your salinity is a tad bit low as well, I run my tanks at natural seawater level of 1.026 but thats not the cause of your troubles either.
I honestly think it's the ammonia build up that is getting your fish. If you try to stock again I would suggest one or two fish max and see where that gets you. I would monitor the ammonia and get the water changes done as quickly as you can if there is another spike. You might have to live with just one or two fish in the tank for awhile though, stocking too fast and too many fish can seriously cause a spike so i would be patient and go slow.
Sorry, thats the best advice I have for you.
Good luck, let us know how it works out.