Natural Heating Source for Aquaponics

I am tired of all the recalls on the food we eat and all the chemicals that are allowed in and on the things we eat. This project is to try raising my own fish and vegetables during winter.  It is a work in progress. I have changed things a few times already, the newest experiment is a natural heating source to raise the temperature of the water in my aquaponics. It is really simple, I placed two sheets of metal under my grow beds. then connected them with a piece of chain. then I filled it with wood chips. Any compost material might work but wood chips don't stink.

What is suppose to happen is the inside of the wood chips should heat up as they start the decomposs process. Being hot air rises it should warm my grow beds and the water in them. this in turn will go back into the fish tank and warm the water there too.