In addition to Dr. Condello's science report above, pH will change more easily or quickly when-
1. Water has less or lower alkalinity
2. Water has more plants present, either phytoplankton or rooted plants.
What Dr. Condello did not mention is that when carbon dioxide is released into the water it forms carbonic acid which lowers the pH esp during the night. During the day the carbonic acid breaks down into bicarbonate.
Carbon dioxide is also always produced 24/7 from bacterial decompositon and animal respiration.
pH is less likely to change when your water is more buffered with limestone either natural or added by you. Ponds in some regions contain naturally hard water (lots of lime). My pond water alkalinity is naturally always around 96-100 ppm (mg/L). Above 20-30 is needed for a good plankton bloom.
