I seem to love my Whisper 30-60. I bought mine at Wal*Mart for $34.97 USD. It is reliable, silent, and the cartridges are cheap. I also like Aqua Clear.
i was thinking about making it a mini african tank have a peacock or two an electric yellow and a pair of johanniis
somethin basic not too special at least for now
Go to Petsmart.com and print out a page for a pengiun 150. Then take it to the store closest to you. At the check out, give the page to the cashier and she will take it from there. It will be cheaper than Walmart and the Penguin has a bio-wheel.
I used a pinguin bio wheel my self and it did a great job. I had the emperor 400 here is a link to it. http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=14448 When I started with cichlids I used that and an undergravel filter for a 55 G and I was fine so it will handle your 20 with out a problem.
They do dig but when they get to the UGF its the same as hitting the bottom of the tank. Maybe it could be a problem if one digs all the way to the UGF and there are fry swimming right over it, but they dont really dig to the point where I ever see the UGF.