I tried making one of those coke bottle filters from the youtube link suggested before, but I used a mini coke bottle, and It does an awesome job. I tested it by adding crushed flakes in the water and it started right away sucking them up into the gravel!
I have a problem though with using it, the sticky glue stuff that is used to hold the wrapper design on the bottle, anyone know what it is? Is it safe to leave it there? Cause it's really hard to get off. I have covered it up with aquarium sealant and it's going to take a long time to dry out. o_o