with such a small tank, why don't you try to make a DIY canister? Pretty cheap to do, although I've never done it myself. Look on-line. Just google "do it yourself canister filter." There's a site that outlines how to make one. Pretty cool actually. All you need is one of those water purifying things that people put under their faucets and a pump (plus some other parts). May be worth a try.
That DIY CO2 bubbler sounds interesting.............what is that, yeast, water, sugar, and an airsupply? Can you just start homebrewing with grain and let the CO2 bubble into your tank? At least, that way, you'd get beer and and a happy tank out of the deal.