Adding pH up without knowing the cause is a bad idea. It is not a permanent fix and the pH will drop rapidly again. Fish will tolerate a steady but high or low pH more readily that a wildly swinging pH which is what you are going to get.
Most likely cause is something in the tank - gravel, driftwood or other common item present in all tanks, overstocking etc.
By the way, 6.0 isn't a crash, merely a drop