This article by Christina Manian, RDN, is worth reading for personal reasons and relates to water quality for animals. Fresh, clean water is foundational to animal health and often overlooked by animal owners <Link>.
When water is left sitting out, uncovered for eight or more hours, the carbon dioxide in the air starts to mix with it. This chemical reaction will result in the formation of weak acid, reducing the pH level of your water. Still safe to drink, its pH is the first culprit of why that water tastes off the next morning.
Source: Better Homes and Garden, August 7, 2024. Link.