I have had a lot of success with a small external filter called TOMS Rapid Mini. I believe this could work for your application.
One suggestion is to get your own media for it as their media that comes stock is a little subpar.
I currently have 3 of them, each one on a separate 20g long Apistogramma tank. The flow they put out is a bit strong so that is another watch out. I got around it by facing the output to the tank wall to disperse the energy.
"What the world needs is more geniuses with humility, there are so few of us left."