Sounds like you have air built up inside the filter when the pump shuts off. the most likely cause is a suction side air leak, bad lid o-ring or union o-ring or maybe even the threaded fitting going into the pump. When you turn the pump off, you may be able to see water squirting out wherever the leak is. Once you find/fix the suction side leak the filter should stay filled with water and not surge backwards when the pump turns off. Make sure you keep the filter clean as well.