Making a Filter for a Small Fish Tank

Needing an inexpensive, small, and effective filter for my White Cloud Mountain Minnows' 2.5 gallon tank (which is really an antique pickle jar inherited from my grandmother), I searched with Google for instructions, finding this article from the Minnesota Aquarium Society. In summary, you can build a combination undergravel and box filter using a funnel, flowerpot (without a drain hole), a bit of filter material, gravel, air line, and an air pump. Being genetically resistant to following instructions, I modified them to fit my situation. My filter went together like this:

1. In the bottom of a squeaky clean Trader Joe's bean dip jar (sans label) from the recycle bin
2. I put a layer of filter material from my other aquarium's filter (like polyester batting) and
   
3. then added a small plastic funnel found in my kitchen, wide part down. The wide part of the funnel should fill the width of the jar but not so tightly that water can't flow through the gap. The narrow end of the funnel should be about twice as wide as the air line so bubbles can easily escape. Mine fit well with only a little shortening that also widened the opening. The point is to get the air into the funnel, so you can put the airline into a second hole in the side of the funnel or, (this is untested), run it down into the filter and up into the funnel but not out the end.
   
4. Then I filled the jar, burying the funnel but leaving the end poking up about 1/4 inch, with clean aquarium gravel.
   
5. Next I filled the jar with declorinated water and put it into the tank. It is nearly invisible because the glass reveals mostly gravel that matches that already in the tank!
   
6. Next I stuck an airline into the funnel and turned on the air, adjusting the flow appropriately.

The minnows and snails are happily investigating their new "boulder," swimming around and climbing on it.