I made the filter for the tank. There are two water returns to the tank; one return 3.5 inches from the left going right, one 3.5 inches from the left going right. There are two reasons for the diffuser being there: 1) the center of the tank is the deepest portion so the bubbles being emitted from the diffuser have time to diffuse before reaching the water surface 2) the diffuser is not that good so it needs a strong flow of water to disperse the c02 bubbles. Most people remove all of their equipment prior to displaying the aquascape, but certainly not before it has had a chance to fill in.