I don't think that having a pump pulling water from your display tank is a good idea. This could easily cause a flood as there is no way to match the gph rating between the two pumps. Even if one pump is at 1 GPH higher than the other, over the course of eight hours or so, a flood would be inevitable. You have to use some kind of overflow system for the display, such as drilling the tank, or a siphon overflow.
You could use a CPR overflow prefilter, such as the one in this link
. In this one, if the power goes out, the air-lifter will re-start the siphon.