Hi, Nguyen!
About pH...I mean this with good intentions
. I think your pH & koi would do much better if you cut your fish volumne by at least half
...hard to do, I know, but like the other folks mentioned the pH will be affected by the amount of ammonia that the fish are putting out and it is hard for the CaC03 (carbonates) to keep up! There is a lot to do to keep koi healthy, and keeping less fish will help the health of the fish and the pond!