High PH-Hal

We are at a loss to figure out why the PH spiked. Nothing new added.our
water is naturally high to begin with. We are in an area where there are
pockets of water too high for fish. ,according to guy at store. He said
he wesn't aware that in our actual area this existed,but there's always
the possibility it could happen.We cleaned bottom thoroughly in the
spring. No rocks or sand in the bottom.Only different thing is that
awful "pea soup" algae. Never had this kind of algae before. Kind that
disintegrates when you try to dip it out!
Question: What is the K H supposed to be? I don't see that test on the
paper I got from store..Hardness is 425.
Was unable to do more than partial change this AM before rainstorm
hit.Guy did say yesterday that water was too high to use buffer.That's
why he advised total change.I think he knows his fish "stuff" but deals
more with aquariums I think. Iris