I made a homemade filter for my 5 gallon... I simply found an empty fish food container, punched a hole in the center of the lid, stuck a tube connector on a tube, put it in through the hole, opened the lid, and added gravel from a cycled tank. You could also add filter floss to this. I then connected the tubing to an air pump.
I careful hid my "filter" in 10 lbs. of gravel. You could get an air diffuser (sp?) to slow down the flow.
No tank should be without a filter, besides betta tanks (since they don't like a strong flow).