Water, as long as it is relatively pure, doesn't really go bad if well stored. People have used cisterns for water storage for thousands of years. Even minor contamination with small amounts of every day crud and dirt tends to be self-correcting as long as oxygen is present and light is limited. While it is best to keep insects and debris out a few leaves or insects are going to be consumed, their nutrients used up and broken down, and the resulting simpler material settles out.

In traditional English plumbing water pressure for the house was maintained by a cistern on the roof or in the attic. This saved wear and tear on the pumps and the need for complicated pressure valves. The cisterns were supposed to be covered loosely and screened. According to literature finding a dead rat, and/or a bird or two, in the cistern was pretty common. Funky as that sounds it was quite rare for anyone to get sick. Of course, being English, they usually took their water in the form of tea or whiskey. Safety from both ends.

In a well maintained cistern, one that excludes the rats and birds, small amounts of bacterial algae and spores are still pretty much impossible to keep out because they come in on the air. If the water they land in doesn't have enough food they die out. If it does, they multiply until they consume the available food supply, and then die out. Their tiny corpses settle to the bottom where they are removed in the regular bottom cleaning that is scheduled ever year or three.