I don't have an answer, but have the same concern for a kalk doser I am trying to set up (also using an aqualifter).
Perhaps a check valve of some sort? Either that, or put a t-connector on the input line, just above the highest possible water line, to break any siphon that gets created? (I am speculating, don't know if either of these is the right answer but they are things I have considered as I ponder how to set my configuration up).