Right, I set the overflow box up at the weekend, and there are a few problems.
The tank is a Juwel 450, the box is wide enough to go over the edge of the tank and the 3d background, but to get the water in the box I have to put more water in the tank, and the brace bars are sitting a couple of mm under water, which disrupts the flow on the surface. Also if I open the feed tap full it cant suck out enough water, and gurgles as the flow is cut short. If I put even more water in the tank it may solve this. Temporarily I have attached the output from a fluidised filter which is supplying enough water to top it up.
I suppose that the box could do with being an inch lower to solve this. Would putting more water in help?