There's a basic principle in water storage.
There's no such thing as emasculate conception in water. Life is not going to magically appear so pure water has an indefinite storage life.
That being said, achieving that pure status, short of heating up everything or using radiation is pretty darn hard. A lot of microbes especially aerobic (no oxygen) come from spores and spores are very difficult to destroy. Now its not hopeless.
What's typically recommended is periodically ( I do twice a year) you jostle your water containers. The idea is to regasify the water, put the gases like oxygen and in the case of bleach chlorine back into the water. It'll make you appreciate the newer storage techniques like bladders but its quite effective.
The real trick is to treat your water like a #10 can of Mountain House. Don't open it till you need to use it then when you do use it all.
The number 1 source of contamination is still exposure and that's simply a way for the germs to get in.
Tj