Don't get the big blue or clear units. You want the 20" tall REGULAR size diameter ones. The big blue is twice as wide and has 1" in/outs.
http://www.discountfilterstore.com/w...lear-pr20.html
This is the one I have. It's the clear one. The blue ones are about $28 iirc. Add a wrench and mounting bracket, as they aren't included at those prices.