Essentially, this filter pumps up water using suction, filters water using gravity, and returns the water by siphon.

It's easy to make, saves room in your tank, and doesn't require a water pump.
You can easily customize it with different gravels, plants, etc...
However, it is pretty slow compared to conventional filters. But slow and steady wins the race.....

Water is forced up an airline tube by the air pump's air pressure.
It empties out into the drip garden, which is a soda bottle with the funnel part inverted and resting on top of the bottom section.

Gravity pulls the water through the gravel (biological filtration), through the sponges (mechanical filtration), and into the bottom collecting section.

Once the water level exceeds the height of the return airline tube's hole in the collection section, the siphon effect drains the excess water back into the aquarium.

in photo: green tube is the air tube. top clear tube is water tube. bottom clear tube is return tube.

Step 1: Materials and Tools

Non-moving-parts water pump from chefmichel

1) air pump
the gentlest Rena or Whisper air pumps are best.
if it's too strong, the bubbles blow out of the wrong tube.
you can find it at the petstore or online.

2) checkvalve
any checkvalve will do. petstore or online

3) T junction.
I'd recommend going to Home depot or Lowe's and picking up an irrigation T junction.
The tubing fits right over them nicely.

4) Airline tubing
twice the length of your aquarium + the length of your soda bottle+ some more just in case.
The more flexible the better.
Preferably clear to see if your pump is working or not.

5) rock

TOOLS: scissors

Filter Garden
1) 2 L soda bottle or 1 L or whatever.
The bigger, the better filtration. But the bigger the heavier. It's up to you.

2) Aquarium Gravel
Fine sand or rough pebbles are okay.

3) aquarium sponge (normal sponges may kill your fish)

4) optional: bio balls or more gravel

TOOLS: heavy duty scissors. Drill and drill bit (slightly smaller than airline tubing).

Also, water. Lots of water. A towel.

Step 2: The Water Pump

The heart of this project is the water pump, which I made from chefmichel 's instructable
https://www.instructables.com/id/Non-moving-parts-water-pump/

To quickly summarize,
1) stick the air tube into the downwards part of the T in the T Junction.
2) stick the water tube into an arm of the T tube. The two tubes should be 90 degrees from each other.
3) stick the other end of the air tube to the check valve.
4) connect the check valve to the air pump with another tube.

5) place the t junction into the aquarium
6) place the other end of the water tube into a cup or something to catch the water.
7) turn on the air pump

in photo: green line is air line. clear tube is water tube. opaque tube is return tube. pagoda is there to keep the t junction from rising as it's mostly air going through there.

water should spurt out of the free end of the water tube.

for troubleshooting, check out chefmichel 's instructable.

Step 3: Making the Filter Garden

1) Take the soda bottle.
2) Make a pilot hole a little below where the funnel part ends so that the diameter of the funnel is the same as the diameter of the collector.
3) Using a scissor, cut all the way around the circumference of the soda bottle so the two sections are now separate .
You may want to smooth out the edges with a dremel tool or something.
5) Invert the funnel part.
6) Remove cap. Save for CO2 reactor.

done.

By the way, if you had cut too generously for the funnel, the funnel may weight more with more gravel. So there may be more stress on the collector section of the soda can, decreasing it's lifespan.

Step 4: Sponge

1) Take sponge.
2) Cut roughly into half.
3) Take smaller half
4) Cut into 4 rectangular strips
5) Take one end of each of the four strips
6) Squeeze and stuff into the nozzle.
7) Invert funnel.
8) Fill the funnel with water
If the flow of water is too slow, remove some sponge strips. You can "thin out" strips by cutting the strips vertically and removing some of the now thinner strips.
Remember, it's better to have a dry garden than one that overflows.
8) Remove water.

Step 5: Funnel Fun

1) take the other half of the sponge
2) cut it roughly into a circle
3) place circular sponge into the funnel. It should be a few inches above the nozzle.
This will keep the gravel from inhibiting water flow. This will also keep the gravel from escaping.

4) fill the space above the sponges with gravel or sand. you may want to plant some aquatic plants that can survive without immersion.

For best results, you'll need Eco-complete substrate. Seachem Flourish fertilizer, and full spectrum fluorescent lights with a timer.
If you don't already have a planted aquarium setup you can mooch off of, it's going to be a hassle.

Step 6: Drilling the Bottom Section

1) Take the bottom section of the soda can.

2) At a height right below where the funnel empties out, drill a hole.

3) The sponge filter requires gravity to pull water down. A water level at or above the nozzle sponges will decrease their effectiveness.
And it's so much cooler to see it drip down.

to see for how to drill a water-tight/air-tight hole.
http://diyyeastco2.blogspot.com/

if you do it right, no sealant required.
the key is to drill a little smaller hole than the tube's diameter.

Step 7: Putting It All Together

1) place the funnel on top of the bottom half
2) brace the bottom half with rubber bands stretched around it near the top.

3) fix the water tube of the water pump to empty its water at the top of the water garden. it tends to move around.
4) place the other end of the outlet tube leading out of the bottom section of the soda bottle into the aquarium

5) start the air pump
Water should flow up and out of the water tube of the water pump into the funnel section.
Then water should flow down through the gravel and sponges and drip into the bottom section.
The water level should then rise until it hits the outlet tube and then drain into the aquarium.

Step 8: Troubleshooting

The T Junction just bubbles and no water is pumped.
+The T Junction is too close to the surface
+The water tube sometimes needs a little priming. Submerge it with water. Then position the free end near the bottom of the aquarium for the siphoning effect to help
+The water and air in the water tube have created alternating sections of totally water and all air. Ideally you want it to be a continuous stream of water below a continuous stream of air. Blow out the water by connecting the water tube to your air pump. Then prime.
+The height of the water filter is too high. Try to curve the tubing like a logarithmic graph. Or lower the filter. Then clear out the water and prime (above two bullet points)
+Your air pump is too strong. Get a regulator or gang valve or something.

The funnel overflows
+remove more sponges

The collector overflows/the return tube doesn't work
+ Make sure no bio balls/gravel is obstructing the tube
+ take the free end of the tube and put it below the filter to start siphoning.
+make sure the pathway of the return tube doesn't rise above the hole in the collector or the water may just backflow.

The return tube leaks
+put aquarium sealent on it
+pull out return tube, seal up with aquarium sealent, and drill another, smaller hole
+start over :(