Hmm.. Sorry about the loss.. But one thing, and it's just my little experience. I stay away from buffers.. It always seemed to be the kiss of death and caused more harm than good.. I'm not sure whether it was the buffer in both your and my cases, but it always seemed like once I added it, something always, always died.
In my experience, it is always better to have a lower, but stable ph. But, I'm heavy on aeration and water flow in the display to hopefully minimize both low ph and also oxygen deprivation. Besides,a ton of flow, fish, along with coral(as long as your not beating them directly to death with it) love it..
You also may want to look into your salt mix, along with, as others said, different tests. That is an extremely low ph level and honestly, I kinda question either the test, or your salts inherent buffering capabilities. Or, something is up as far as maybe co2 holding in your tank water.. Simple way to test, is to run a fan on the surface of the water and open your Windows. If the ph goes up, holding co2 and it needs more fresh air. You can also, in your fuge, throw more light on it for your macro, to take out co2(along with nuisance elements)..
But, there are a lot of possibilities with what could have hastened his life. But if all the other fish are good, I lean towards it was just that animal. But just watch the other fish, along with maybe feeding them some medicated/vitamin enriched food for a couple weeks..
Best of luck..