Here's a link that explains nitrification/denitirification with enough technical info while keeping it easy to understand. It is refering to ponds, but the mechanics are the same in a SW aquarium.

http://www.fishdoc.co.uk/filtration/nitrification.htm

Running any kind of mechanical filtration without cleaning it weekly or more often in nitrate-troubled tanks will cause the contraption to definitely become a nitrate factory.

I use Hagen powerheads with Quickfilter attachments filled with foam in all my reefs. The powerheads are removed, broken down completely and cleaned during the weekly water change. Foams are usually pretty dirty even after only a week. I have 0 ppm nitrate in all of these reefs, some of which are heavily fed, but only because I clean weekly.

I have a 55 gallon tank with 45lbs of LR. Does this mean I don't need to run the Fluval? I like it for clearing the water as well as using Phosguard and carbon. I Change the floss, phosguard and carbon weekly. I also use a Berlin turbo skimmer in my sump. I have sponges in the overflow box to silence it and to stop any large debris from entering the sump. I do have a few soft corals and a few fish.