[nereefpat]

So the drain works by a syphon from the display tank to the sump?

What happens if the syphon breaks or is plugged? Will your display tank overflow from water being pumped from the sump?

I suppose I would go sumpless in your case, until you can buy or design a better drain system. A design could be as easy as drilling one hole, then adding some pvc pipe and fittings (really cheap).