If you've ever wanted to try Aquaponics, growing fish, plants and bacteria in a nearly-closed loop ecological system, you can get your feet wet by building a basic system using IKEA Antonius frame and wire baskets, two plastic bins, PVC pipe and a submersible water pump.
You'll also need to create a bell siphon, which you can build from PVC components in the full instructions list from aquaponics store Japan Aquaponics's weblog. The structure of the system comes from assembling the IKEA frame as per the instructions and using the wire baskets to hold a 25L bin for the grow tray and the bottom 50L bin to house the fish.
You'll then create a bell siphon from PVC parts and assemble the plumbing from PVC pipes. You can then add water to the tank and test out the operation of the system. If all is go you then add a growing medium such as perlite, lava rock, or river stones to the growbed and add fish and plant seeds.
For further instructions on making the system and on aquaponics in general check out the source link below. This seems like a great starter kit if you're interested in the subject and it's much more productive than most pet aquariums.