This is more in response to Brangdon's question, but the filtering could be set up on the output end, rather than the input. So contaminants from outside, or anything growing inside your tank, would get filtered as you used the water.

In general, I think it's a good idea to purify stored water at the time of use anyway, although it's probably not necessary in the majority of cases if you've taken some care in storing the water initially.