gravity is good. the way mine is set up, I cannot over flow my sumps when doing a water change either because they level off with my make up water barrel.
Those totes are great and I originally purchased them to move the rock when I bought the tank. They were very handy to fill with rock and water, and then roll onto the truck. BUT, they will not last forever and are showing some nice bulging now! I got more than two years out of them though so I am really happy with the way it worked out.