I have the same overflow and had the exact same problem. They packed the wrong tubes with mine. The interior side was way shorter then the exterior. Two possible fixes here. GLUE in risers on the exterior to raise the level in back above the interior tube height or get the right tubes. Sounds like you have the right tube now. As far as not handleing 1200 in flow...What is your plumbing out of the box, maybe that is the culprit. OH Yea, what is diameter of the tubes you did get it working with? 1 1/8 less flow then the 1 1/4 they are spose to work with.