I made a hamster powered aquarium pump. The hamster runs on a wheel, turning the axle and a second wheel also fixed to the axle but hanging outside the cage and partially immersed in an aquarium. The spinning wire mesh of the wheel creates a current in the aquarium thereby oxygenating the water and preventing stagnation. No glue was used during assembly, the only tool I used was a file (to make the holes in the exercise wheels slightly larger). If it rusts, I can paint it with latex or any "food safe" paint. Materials used: two 10 gallon aquariums, a "tank topper" cage, two 7 inch exercise wheels, a threaded rod, nuts, washers (two sizes), small diameter plastic pipe, and dental floss.
The 3 inch wide by 7 inch diameter wheel intersects the water over a length of about 3.5 inches, and is submerged to a depth of about .25 inches. With minimal effort I was able to spin the exercise wheel inside the aquarium "tank topper" and produce a significant current inside the pot of water. In addition, the wire mesh of the exercise wheel produced small bubbles in the water. If this wheel was similarly placed in a ten gallon aquarium at the same depth, I believe a running hamster could produce an above average rate of flow when compared to that of most aquarium pumps. Problems with inconsistent operation could be solvable by setting up several cages in tandem, but let's not get ridiculous! (As opposed to what I am describing here, which is of course entirely sane.)
To imagine this set up in its final state, you have to picture the tank topper on a ten gallon aquarium, complete with hamster and accessories. Sitting parallel to and beside the hamster enclosure is another ten gallon tank, elevated slightly to allow the wheel to intersect the surface of the water as described above. Approximately centered in the water, the spinning wheel would produce a current regardless of which direction it is turning. The aquarium could house several small and hardy fish, invertebrates, or other organisms.
See additional photos and drawings of previous designs: photo, photo, drawing, and drawing. (Note that in the photos the wheel is oriented perpendicular to the longest dimension of the aquarium - current plans call for parallel orientation of the wheel to improve water flow.)