I've got an Edge. The filter is actually very good, (Basically its an Aquaclear 20) and is way capable of filtering 6 gallons of water to perfection.
My advice is to make sure you've got the sponge oriented correctly in the media basket thing, so that it is snug against the sides and water can't bypass it very much. If all else fails, get a bag of filter floss and stuff some in between the sponge and the carbon, or perhaps beneath the ceramic media.
Also, try a small foam pre-filter on the intake.
Oh, and keep on eye on the filter floss if you use some. It can get loaded with crud and block the flow, eventually.