I bought pool test strips that go from 6.8 to 8.4 ph and test last
night it was at least 8.4 ph

Koi prefer 7-7.6 ph.

I read that best ph is 7.0 for fish?

There is no universal ph for fish. A ph of 7 is approximately neutral.
Marine fishes prefer a ph of 8.2-8.4 or so. Koi can tolerate higher ph
than the listed range.

What would cause this, besides concrete?

Your tap water is the most likely cause. Test your ph right out of the tap.

How old is your pond? How many fish do you have, and how young are they?

C//