My anecdotal response. I don't own the shadow but my over flow box made by Modular Marine is very similar. I could not get to to stop leaking until I replaced the gaskets that came with the box. I could not see anything wrong with the gaskets they sent me, I drilled the tank (acrylic) and made sure the holes were smooth, debris free etc. still no luck. After my third time of draining the tank to the level required and in my desperation I decided to replace them with some gaskets from some Lifeguard Aquatics fittings I had not used. I hand tightened then put a 1/4-1/2" turn with the wrench and voila no leaks.