Salt... The 90 on top of the vent pipe serves 2 purposes. 1. - It 'hangs' the overflow on the back of the tank well, by the 2 contact points. 2. It's not glued, and I can take the cap with the whole in it off and turn it facing up and poiur water in it to get the water in the bottom loop before yhou start your siphon. It has nothing to do with back flow in this case. I'll make a few comments on your next post there.