firebirdude
User and Abuser
First, think about how a return pump works. It pumps water into the display, which causes water to overflow to the rear of the tank.... equal to the amount pumped in obviously. This is how any return pump works in any system. So, the flow rate of the return pump will not change the water level in the sump. Ever. Provided the pump output doesn't exceed the overflow rate (which it's not even remotely close in the biocube). So if your stock pump wasn't pumping at the designed rate, the overflow just wouldn't have as much flow through it. The water level in the rear would never change due to this.
Flow through your filtration system should be low and slow. Any granular media will work better this way. On the flip side, you do want a decent amount of flow in the display. Use powerheads. I think most people try to upgrade the return pump to increase flow in the display. It does of course, but not much. The return pump is just designed to return water from the filtration system. Display flow would be a bonus, but in the Biocube, you don't have the option of regulating filtration flow any other way (as with those with external media reactors do, for example). In short, leave the stock pump.
