So I was looking for a way to get rid of some surface scum from my 20L that doesn't have a sump. My sister gave me some stuff that she had laying around for her betta tanks, and I decided to play with some of it.
One of the items that she gave me was a little Whisper filter. There's a small powerhead that was attached to the bottom and pumped water into the filter and up and through the filter material.
I decided to reverse the flow and pull water DOWN through the filter and through the bottom. I had some goofy pump parts laying around and I rigged it so that instead of the little pump pushing water in the bottom and through the filter and out the top, a spare maxi-jet draws water in through the top and then out the bottom of the filter housing.
Here's a shot from the back...
Seems to be doing a pretty good job so far!