Aquaponics is a relatively new idea where you farm both fish and plants in the same set up. Typically you have a large tank where you raise fish for eating. The water circulates through a bed of plants. The fish waste provides nitrogen for the plants and the plants help provide food for the fish. It isn't a perfect system but you still need to add water and feed the fish, but it is very efficient I can't wait to have a greenhouse where I can set up one. I would love to grow citrus fruits in the winter and be able to harvest delicious tilapia. However like I already said apartment living is not very conducive to it.
So I improvise!
Here is my 20 gallon fish tank, I keep a few tropical fish in here. I pull about 5 gallons ever 2 weeks and pour it on my plants. It gives the plants a nice little boost and recycles the water I would otherwise just waste. Less water I have to pay for and the bonus. The cats get awesome "Kitty TV"
If you would like to learn more about aquaponics here is an excellent TED talk