Actually, it's a capillary action that draws the water up. A cheap check valve will maintain air pressure in the line to prevent this and allow you to put the pump on the floor where the air is coolest. Another option is to use a larger sized hose than 1/4" then reduce it at the airstone. Or just skip it. I've had this happen in my fish tank before, but never in a DWC environment. I've turned off my air pumps countless times with the buckets full for one reason or another and nothing bad has ever happened. I don't use check valves on anything but the fish tank now... but hey, it's good, cheap insurance. Maybe I just talked myself into putting one back in line. =) Perhaps the water surface tension of fish tank water is higher(or would that be lower?), but I can't say for sure.