Generally pH is not something we have to worry about in our ponds. Our New Orleans water pH is so high...about 9....and I never worry about it. Sometimes in a new unsealed concrete pond, lime can leach out and pH skyrockets to unsafe levels. There are commercial products you can use to bring it down. Usually they are called pH decreaser or pH down.
BTW, pH is a measure of how acid or alkaline our water is. A high reading or number means alkaline, a low one means acid. About 7 is ideal.
I have never found a pond with a too low pH reading, but baking soda would probably fix that.
When I find a pH level that's too high, I usually use vinegar to lower it. That can make the neighbors think that you are either dying Easter Eggs or making a giant Caesar salad in the back yard, but it will do the trick. Some folks use muriatic acid, but I am too chicken for that.
But usually if you have the proper amount of submerged
vegetation and floating vegetation, the pH will straighten itself out.