Return pump as a source of flow?

[viperfish]

Just be sure to watch for dead spots where waste will accumulate. You lose some of the flexibility of powerheads when your return lines are your only source of flow unless you are using a flow director which I highly doubt on a tank this size. Installing ball valves on each return line (assuming you are using more than one) allows more control of the flow on each side. A SWCD is another option for this setup, it will give you an alternating flow pattern.