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The fluval 306 and 406 are on sale for 118.99 (306) and 169.99 (406) from Petco online. This is also with Free shipping for any order over 50$.

The fluval 306 is good for filtering up to a 40g tank.
The fluval 406 is good for filtering up to a 55g tank.
(See filter reference viewtopic.php?f=9&t=30145#p288939 )

Here's the online link to the filters:
http://www.petco.com/product/117231/Flu ... lters.aspx

I hope this link works as I'm on a mobile device, and hope this helps anyone on a tight budget.
Good luck!

It's a bit pricey, but good to have options. If you're in the range for the 406, look into the FX5 as well.