I have a Koi pond and I learned a lot about maintaining water quality for my friends, the fish. I have acidic well water, which is a concern for me and the fish.

When I first filled my pond which is approximately (10'x25'x4') in size. the water pH was 6. The day after the pH was 6.5, and the next day it was 7. The pH topped out at 7.5. The moral to this story is if you fill your own water vessels with water and let it sit for a few days ( I cover mine with cheese cloth to allow air penetration) the pH improves. I use a wide range pH indicated solution which comes with a vile you fill with water instead of strips.