I just brought home the Aqueon Proflex Model 3 Sump for our new 90 gallon Megaflow Aquarium. We will be setting it up for the Refugium Method of filtration and have a question about the "adjustable wall". The instructions tell you to slide the adjustable wall to the top of chamber 1. The problem is there is nothing included to hold the adjustable wall in place. Any suggestions?? Thanks.