It is not hard to convert your backyard pond into a flourishing aquaponics system. The basics are most likely already in place, and you just don't know it. The foundation of a system like this is to create a balance between plant life and animal life in your pond.
Step 1 - Weather
The first thing to determine is what kind of local weather you have that will effect your aquaponics system over the course of a year. You want the pond deep enough so that if it freezes in winter and fish have room to live under the ice. A depth of 6 feet is safe for freezing and will allow room for them to live.
Step 2 - Choosing Lifeforms
Pick out plants and fish for your aquaponics system that will thrive in your region and not be effected by your climate. You do not want to use plant fertilizers in or around your pond, as it can contaminate the water and kill your fish. Coy fish and Lilly's are great for outdoor ponds and can generally survive winter environments.
Step 3 - Environmental Balance
Once the basic system is setup, your plants should be mainly touching the water and getting the rich nutrients from the fish waste, which can be broken down in your aquaponics system using air stones and water pumps to regulate water flows and the waters nutrients. A balance of plants and animal is essential.