you can always make your own internal air-powered filter. Take a plastic container (I like the ones they sell vitamins such as omega 3 in) and drill lots of little holes in the base - make it like a colander. Drill a hole in the lid that snugly fits some airline, and also colander-ify the lid. Fit airline tubing so that it goes down to the base then fill with your choice of filter material eg carbon (put in pantyhose first), bionoods, zeolite, filterwool. I have seen chopped plastic straws used in lieu of bionoods, and aquarium gravel makes a suitable biofiltration bed too. If you've done it right, air goes down the tube to the bottom, then percolates back up through the filter material dragging water with it as it goes and back up out the holes in the top of the lid. Easy peasy and cheap enough for scrooge. And you can make whatever size/shape you want.