Two years ago we built a solar greenhouse. The beauty of the greenhouse has been fresh tomatoes in January, a quiet place to escape and a learning space about gardening and nature. This year we decided we would add an aquaponics system to the greenhouse.
What is aquaponics?
Aquaponics is the marriage of aquaculture (the raising of fish for eating) and hydroponics (the raising of plants in water). The beauty is that it creates a sustainable closed system. The water from the raising of fish is normally a waste product but is actually filled with nutrients that provide what plants need for growing. Rather than having to feed the plants with inorganic nutrients, which is the normal practice in hydroponics, the plants utilize the "waste" from the fish.
Our goal was to grow tilapia and vegetables in the system. Our greenhouse has a floor footprint of 10'x 15'. For a fish tank, we used a pallet tank which is approximately 4'x4'x4' and holds 275 gallons of water. We dug a hole and put 4" of lava rock in the bottom to insulate from the earth. We then placed the tank in the hole and put foam insulation around all of the sides and metal sheeting outside the foam insulation. This should keep the earth from drawing out the heat from the water in the tank.
With the tank in place, we cut 55 gallon blue barrels in half lenghtwise and placed them on buckets filled with water for grow beds. We later had to provide further supports for the barrel-halves as they do tend to buckle. If we had it to do over again, we'd build a frame to hold the barrels but still place the buckets underneath. These provide a heat sink to help moderate temperatures in the greenhouse.
We then plumbed the system and filled the grow beds with gravel. I planted our citrus trees, broccoli that had been pulled from the original greenhouse bed and some seeds. Then we waited for fish. To help feed our plants we added fish emulsion to the water. My theory was that it would help to populate the system with the bacteria needed to convert the eventual fish waste into nutrients for the plants.
When we were ready for fish we started working with another individual who was also building a system to try and get tilapia. While we waited, we added several goldfish from the local pet store. We watched as these fish died along with a second batch. We then learned that the feeder fish you purchase at the store are often not healthy and that was the likely reason for our dying fish.
Our next effort to add fish was purchasing several tilapia from the local Asian fish market. Those turned out to be unhealthy as well and we watched in horror as they started to die. We got some great advice to turn up the water temperature to 85 degrees F and add salt to the water. We were able to save five of the fish who provided nutrients for our plants until we could get more. It took another two months before we were able to get tilapia fingerlings into our tank. When they arrived, we celebrated by eating the large tilapia from the tank.