Aqualifter question

[macky]

I recently bought an aqualifter to use as a top off pump. I have a pail holding fresh water on the floor and the pump is located at a height higher than the water level of my sump. One of the lines is in the pail of water and the other is in the sump. The aqualifter comes on twice a day with a timer and has worked fine for 3 weeks. Yesterday I found it siphoning water from the sump to the fresh water pail when it was turned off. What am I doing wrong?
Thanks