As for the reservoir, most people in a recirculating DWC setup seem to keep them at the same level because by monitoring your reservoir level, you simultaneously monitor your DWC buckets levels. My friend's uses a recirculating DWC, but the setup is a little different. The reservoir sits on the ground, pump inside pushes nutrients up to the tubs on a counter and the excess drains back to the reservoir... make sense?

As for the liquid karma question, most people add it when they completely change out their nutrients, not at top-offs, and you want to add enough for the entire volume of your nutrient solution.