Pure water is made up of nothing except hydrogen and oxygen. This means that unlike foods, water itself cannot go bad. Food eventually breaks down, if water breaks down you simply get hydrogen and oxygen gas. Water can become unusable, however, if it becomes contaminated. Good water stored correctly in a sealed and sanitized container, water will have an unlimited shelf life.
Water can only spoil if something gets in it or if something like algae or bacteria were already in the water and have multiplied to unhealthy levels.
You can also help the shelf life by storing any water in a cool, dark place. You need your water to be clean and free of contaminants and the storage container must also be free of molds, bacteria and other unpleasant contaminents. So, for now filtered tap water is fine. Later boiled water, or chemically sanitized water will be needed. Water that is simply filtered can be used, but just filtering it might not be sufficient for long-term storage if the source is rather dirty. A store of chlorine or bleach goes a long way in preparing water for long term storage.
Let's talk a minute about storage containers. The container can be as small as a simple store bought plastic bottle of water or as large as vessel that can store 1000s of gallons of water. Plastic bottles are fine, but be aware that their thin walls break down over time and plastic particles in the water acts like estrogen in your body. Not healthy, but also not something I would be overly concerned within a survival situation.
That said, this is why it is better to get large thicker walled plastic containers. The buildup of plastic in the water will be slower due to the volume of water compared to the amount of plastic contact it has.
So how big or small should your storage containers be. That is a person's choice that will depend in part on your situation. I personally think that 5 gallon containers are the best choice, with a small store of plastic bottles that can be thrown quickly in a backpack if a hasty exit is required. The 5 gallon containers are cheap and are small enough to move around or even transport long distances. If you get larger than 5 gallons transport quickly becomes a huge issue.
Even though water can last forever, it is a good idea to rotate out your water periodically. The longer it sits the great its chances of being unhealthy. So, while times are good it makes sense to rotate your water so you not be caught with water that is bad or is about to go bad that you were counting on.
Water that has been contaminated could have a smell, start to have a tint of color or become cloudy. If so immediately dispose of the water and clean the container if it is to be reused.
Water that has been stored and is perfectly healthy to drink can actually have a bad taste. If your water has a funny taste, try shaking the water vigorously to oxygenate the water and see if that fixes the issue. Dissolved oxygen is normal in our water and the lack of oxygen of stored water can cause the bad taste. It might take quite a bit of shaking. A couple of shakes isn't going to do it.
Water that has turned black probably means mold. Drinking this water is never an option. This can kill you. Don't risk even a sip. And that container should be discarded or thoroughly cleaned if glass. If plastic... will don't risk it if you can avoid doing so.
If you would like to learn more about emergency water then take a look here: (
Primitive Water Storage And Collection) I have articles on water sources, filtering water, collecting water and more.