I have similar tanks and take the approach - if it ain't broke, don't fix it. Unless there is something in there keeping the water from making it to the tap I treat it to make it safely potable and fill it from known water supplies.
The great thing about having two tanks is that you can batch treat each tank of water and prepare it for use. Using one tank at a time, and only taking on as much water as you will use in a week or two. Here is a very useful calculator to help you determine an accurate chlorine treatment regime using common unscented bleach to both shock, and maintain a residual chlorine content - effectively sanitizing the water.
https://public.health.oregon.gov/He...ater/Operations/Documents/ShockChloroCalc.xls
A shock will effectively oxidize the organics and dissipate, set a 1-2 ppm (mg/l) concentration to keep the water sanitized. A POU (point of use) carbon filter will remove the chlorine taste/smell.
With my 70 gal tanks the math works out to about a shot-glass each of 5% bleach to get them to 1 ppm chlorine.
