You have to seal the path between the inside box and outside box so the water can only pass between the interiors of these 2 boxes.Am I missing something? Wouldn't you just want the gasket on the wet side? Why 2 gaskets?
If you only had the gasket on the "wet" side (between internal overflow and tank wall) then water could get around the nut on the inside of the external box, through the hole, then down the outside of the tank wall.


