How it works:
Fish are raised in a tank (at a humane ratio of 1 fish per 2-5 gallons of water)
Water is pumped from the fish tank into a soil-less plant growing area
Bacteria convert accumulated fish waste (toxic for fish in large quantities) into plant food
Plants absorb the nitrified fish waste
Water is constantly being filtered by plant roots to keep it clean
The cycle repeats indefinitely.
The only input into an aquaponics system is food which the fish consume, resulting in a completely organic system. As the fish grow and the system ages, the number and variety of crops you can grow also increase so long as you maintain a neutral pH, maintain high oxygen levels, and honor temperature requirements for both fish and plants.