Where is it leaking from? I have one, and also found it awkward to setup.
There is only one gasket on mine, which is designed to go between the outside of the tank and the outside of the external box. This is with the idea that water can then flow very slowly along the threading between the two boxes, but the gasket is supposed to keep the water from leaking between the two pieces (and along the back of the tank).
While that worked for me, it did result in a slow leak into the inside overflow box, and thus down the overflow eventually. It would admittedly have taken a day or two, but eventually my sump would have overflowed if I were to let the tank drain to the bottom of the overflow bulkheads like this.
I personally choose to silicone the bulkheads into place. I put silicone between the threads and the inside hole of the tank, as well as silicone on the inside flange of the bulkhead to the interior box. This keeps water from flowing into the box and down the tube, as well as helps prevent water from flowing along the threads of the bulkhead.
I wish it just had two additional gaskets, one inside the interior overflow box, and one between the inside of the aquarium and the overflow box. I considered trying to just find and add those gaskets, but so far silicone seems to be an easier solution if you can stand the top half of the aquarium being dry for > 24 hours.
