Tank flow question

[Greg Depo]

So this might be a newbie question but I was curious how does our tanks filter ALL the water in the tank? Currently I have a return at the top of my tank and the over flow is in the center at the top. I have two icecap 2ks at the top on each side
of the tank. To me I would assume that water at the very bottom of the tank would be least likely to be filtered through the sump?? Is this assumption not correct? If it could be accurate, is there a better way to ensure that my water flow is better setup?

 

[reefwiser]

The water in an aquarium goes in circles. As it hits the sides of the tank the water goes down and will washes over the bottom of the tank. You want to get this circular flow going so that the water that goes in the overflow skims the top layer of the water.