float switch ... check valve ... pump
use a float switch to turn off pump when barrel with aged water becomes full. otherwise, pump moves water into aging barrel automatically. check valve used to keep water flowing in one direction only even when pump turned off and aging barrel has more water in it than others. you can daisy-chain as many barrels together like this that you want.
if you're worried about the float switch failing and creating a flood, consider using something like this:
http://autotopoff.com/products/DT1/