$0.00-60.00 - 3-10 gallon fish tank - http://www.amazon.com/Back-Roots-Aqua-Farm/dp/B00CN52TRM/ref=sr_1_1
$7.00 - 4 Gatoraide or similar bottles (I plan to eventually double this with two hanging pot strands) - Convenience store
$3.00 - Air Tubing Connectors - http://www.amazon.com/gp/product/B0051SM6GW/ref=oh_details_o02_s00_i00
$15.00 - 40+ Gallon Air Pump - http://www.amazon.com/gp/product/B0009YF4FI/ref=oh_details_o03_s00_i00
$5.50 - Air Tubing - http://www.amazon.com/gp/product/B0002563MW/ref=oh_details_o03_s00_i02
$4.00 - 3/4" or larger PVC Pipe - Hardware Store
$2.00 - 2 Elbow joints, 2 T joints, 4 caps/straight through connectors for PVC - Hardware store
$3.00 - Synthetic Ropes - Hardware Store
$2.50 - 4 way gang valve with adjustment nozzles - http://www.amazon.com/gp/product/B001CQ69I8/ref=oh_details_o03_s00_i01
$4.50 - Check valve - http://www.amazon.com/gp/product/B005HTT1P4/ref=oh_details_o02_s00_i01
Dremel with cutting bit
Leatherman or other small saw
Step 1: Get Bottles, empty, and clean
Step 2: Cut Bottles
You will need to cut a hole into the base of all the bottles except the top one. Make the hole small and slowly work it larger and larger until the top of the next bottle will fit into the hole in the base of the bottle below it. You don't want it too big as the cap of the upper bottle should hold the lower bottle up.
You will then need to cut holes into the sides of the bottles that are large enough to fit rocks and plants into. I did mine on either side of the bottles and used the lines on the bottles as cutting guides.
Next you will need to cut holes into the lids of the Gatoraide bottles to allow the air tubing to run from the bottom of the assembly to the top. I also added 4 draining holes to each of the lids although you likely won't need them as the air tubing shouldn't completely seal the hole it's running through.
Step 3: Make Stand
To make the PVC stand, you will need:
2 lengths of 3/4" or larger PVC pipe (I'm not sure how long the lengths at the hardware store are, but they're uniform in all the stores I've visited)
2 T connectors that fit the pipe
2 elbow joints that fit the pipe
4 caps or straight through connectors (they were out of caps at the store I visited but found the passthroughs to work fine to steady the base)
Total cost of PVC/Connectors ~$6
I estimated the vertical posts of the PVC tubing first with a measuring tape while holding my Gatoraide bottle assembly above my fish tank. It came out to 43" so I cut two tube lengths from the PVC. I then measured across the fish tank and made a 14" tube to be the top bar. I assembled the three pieces with the two T connectors at the base and the elbow joints at the top and held it up to my fish tank to measure how long the feet should be. I came up with 5" segments for these and attached them to either side of the T connector and then attached the straight through connector to them to make the assembly level. Unless you're using the same sized fish tank as I am, you will likely have to adjust these measurements to fit your tank and bottle assembly.
2 - 43"
1 - 14"
4 - 5"
Step 4: Hang Bottle Assembly
Tie the top bottle to the PVC pipe or hook so that the assembly hangs over the fish tank at the right height. Then, push a piece of aquarium tubing all the way from the bottom bottle cap up to about level with the bottom of the side holes on the top bottle. Leave enough hanging out of the bottom of the bottle so that it will be able to go approx halfway down the water level in the tank.
Run the tubing through the hole in the top of the growing bed and then connect an aquarium tubing T valve to the base of the tubing. Add another length approximately 1-1.5 feet to the parallel side of the T valve inside the aquarium. This length will sit along the bottom of the tank and suck up water for the assembly above.
Fill the Gatoraide bottles with rocks or clay pellets for the plants to grow on up to the base of the holes on the side.
Finally, attach an air tube to the 90 degree connector on the T valve and run it to your air pump. I recommend using a check valve here just in case water gets in the tubes so that you don't fry your pump.