In both of my tanks (both 20L) I use a 10i-20i internal Tetra Whisper filter. On one, I have plastic bottle baffles on both intake and output. On the other I used pantyhose and a rubber band. Both do their job well, and with the baffle, they don't create a lot of current.
I like the option of having carbon, as it clears the water. It also has a place for biological filtration to occur (sponge or bioscrubbers that increase surface area for good bacteria to grow)
They were about $15-$20, you can get them on Amazon, or Walmart. This is the link to the exact one I use
http://www.amazon.com/Tetra-Whisper...id=1395495717&sr=8-1&keywords=whisper+10i-20i
By being internal, you save that extra space behind the tank, so it can be closer to the wall. I actually have one hanging on a side instead of the back, so that I can see them from the front and back (its on a pass through between my kitchen and livingroom).
Some people don't like HOB (hang on back) filters, they prefer canister filters. On a canister filter you can purchase a spray bar that creates less current, but they are pretty pricey. The same effect can be found by using baffles.
Others say don't use internals because they may raise the tank temp. I can tell you that if they do, it is so minimal that it doesn't register on either of my thermometers.
It really comes down to personal preference, and price point. I have always had luck with this particular brand when I kept freshwater fish so why change now.
Hope that helps, lotls of luck
