buff means buffer. But I would not add buffer until you have an alkalinity and calcium measurement. If these have not been addressed, then you can cause MAJOR problems by just adding buffer.
pH problems can be caused by several factors.
IMO, 7.9 is not critical if it is stable. I've tested ocean water at 7.8.
But I really can not emphasize enough that you should have your parameters rechecked, and also get alkalinity and calcium done as well.