My personal view is that if you are storing clean water in a clean container and storing it properly, the risk of anything growing in your water is very, very small. And even smaller if you are changing the water at six month intervals. Properly stored water should have few microorganisms to start with, and no nutrients to sustain them in storage, let alone promote an increase in their population.

However, I think it is a good practice--just to be safe--to treat the water as you use it. In other words, I don't treat my water when I store it. However, if I do have a reason to start drinking my stored water, then I will treat it (my choice is bleach) at that time. But I have had water that was stored for a couple years straight from the container with no problems, too.