I have used a Marineland Penguin bio-wheel 200 in my 29g tank for the past few years and it has been good to me. It does a great job of keeping the water crystal clear despite a heavy bio-load.
The replacement cartridges are about $8 for 3 of them - so I bought bulk foam/floss and charcoal from my LFS
, and make my own to save $
46 Gallon Bow F/W: 4 - Zebra Danio's, 2 - Red Platy's, 2 - lg. Silver Dollars, 1 - Pleco (4.5"), 1- Gourami, 1 - Cory Cat, 1 - mystery snail, 4 - Cherry Barbs, 1 - Black Molly