Maintain a compost heap
A compost heap will generate heat to keep our water in the aquaponics setup wat a certain temperature. This is also a good breeding place for worms, which we can use to feed the fish.
How to start a compost heap?
Have a look at a video from WWII about making a compost heap http://www.youtube.com/watch?v=Ce7IVl-glaQ :)
Jean Pain (1930–1981) was a Swiss innovator who developed a compost-based bioenergy system that produced 100% of his energy needs. He heated water to 60 degrees celsius at a rate of 4 litres a minute which he used for washing and heating. He also distilled enough methane to run an electricity generator, cooking elements, and power his truck. This method of creating usable energy from composting materials has come to be known as Jean Pain Composting, or the Jean Pain Method.
See http://www.youtube.com/watch?v=bVCaczil4W4 for an old video of Jean pain showing his system.