hi sphinxlover,
i am not clear what is the 'air blow from the sponge' but the carbon filter for fish tanks are in range of RM8, RM15, RM30 depending on it's size of packaging and places. for a 20 gallon fish tank, correct me if i'm wrong it's about 1.5 feet? this tank will just have to use the smallest package of carbon filter. there are a lot of carbon filter brands but i think the most cheapest one is enough to do the trick.