I'd keep the magnum and use it for chemical filtration (carbon, purigen, etc). I'd then supplement the flow with a couple powerheads as your friend suggested. The powerheads just make current. You want random currents, versus one big circular current - like you'd get with just the output of the magnum. A good way to create random currents is to put a powerhead in the back corners of the tank, pointing towards the front center, kind of in a "V" pattern looking down from the top. Depending on your rockwork, you can bounce the flow from powerheads off your rockwork, or do bank shots off the side glass - anything to break up the flow and make it more chaotic. It'll take some experimenting but after a few tries you'll find something that works.