Yikes, glad you caught it. When I installed my auto topoff I was able to keep a close eye on it for a couple of days and noticed that it was syphoning on its own. I quickly realized that if the tube was pointing down when the pump turned on that it would simply continue to syphon when the pump turned off because when it stopped air was not able to get into the tube. I simply ziptied the tube so that it pointed up and this made it so a air bubble shot up the tube and stopped the flow; I never had the syphon problem again. And my water level in the resivoir was higher than that of the sump with no problems at all.