just build a sponge filter, its the easiest and most simple filter i know of.
Use a open cell foam, then put it into a container (that can be put in a freezer) and fill it with water and let it freeze overknight. the take it out and cout a a shape and in the center of the use a drill or something to get a hole as wide as a air line tube about half way. then stick in the airline tube and attach it to the air pump. As simple a that. But im not sure if thats the exact procedure because i don' really got it memorized.