This is a very good point! As a result of adding more water to your sump to compensate for the needed water in the pipes, your system will drain this extra water into the sump when there is a power failure. There may also be some water siphoned from your display tank via the return outlet. Definitely make sure to test a power failure now while your in front of the tank to either make sure your sump can handle the extra water (due to power failure) or that your check valves are working (if you installed any).

I ran a test last night and the sump held! The size of the sump is 26x26 cube with the height at 16" If it did start to over flow I have a 5 gallon side that fresh water will be held for my ATO. I will keep this about half full just for the reason of over flow but all seems good. I will do a test run again tonight just to make sure.

My returns are not pointed down but near the top so the back fill shouldnt be a ton..