When I visited Rob Torcellini in his Connecticut backyard, he took me inside his 10-by-12-foot greenhouse and showed me how his plants grow so tall they curl around the ceiling. He explained to me his gardening advantage: no soil, continuously circulating water and fish poo as fertilizer. (For anyone unfamiliar with fish waste, there's no need to fear E.coli since fish are cold-blooded they don't carry the bacteria.)
Torcellini is an accidental aquaponic gardener. He originally bought a $700 greenhouse kit to grow more vegetables in his backyard. Then he added fish to get rid of a mosquito problem and before long he had torn up his greenhouse to add pipes, pumps and grow beds to create a complete aquaponic system.
Fish poo as fertilizer
"The fish excrete ammonia through their gils as their waste and that ammonia travels in the water and gets pumped into the growbeds," he explained, pointing to his homemade planters currently filling with water from his fish tanks, "and there's a naturally occurring bacteria that converts the ammonia into nitrites and then the nitrates and then the nitrates are absorbed by the plants as a fertilizer. So it's a whole natural process that breaks it down."
You don't have to understand the chemistry to grow this way. There are thousands of Americans (judging from the popular online communities), and even more Australians (it's popular in this drought-prone country because aquaponics uses 80-90% less water than traditional agriculture) who are growing fish in a symbiotic environment with their vegetables.
Aquaculture + hydroponics
Aquaponics combines fish farming (aquaculture) with the practice of raising plants in water (hydroponics). There is no soil; the plants sit directly in gravel that is continually flushed with water, and fish waste.
It's organic by definition: instead of using chemical fertilizers, plants are fertilized by the fish poo (and pesticides/herbicides can't be introduced to kill pests because they could harm the fish).
Since the plants don't need dirt, aquaponics allows gardeners to produce more food in less space. And in addition to the vegetables they can grow, most aquaponics gardeners cultivate edible fish as well.
Backyard fish farming
The most popular choice of fish is Tilapia because it's breeds well, grows fast and can survive in poor water. Though they are a warm water fish so for people in colder climates -- like Torcellini, who lives in Eastford, Connecticutt -- it's not a great option.
Torcellini is currently farming goldfish and some koi, but he explains that if he wanted to grow edible fish, he could switch to a cold water fish like trout or perch.
DIY gardening
Aquaponics is being done on a more massive scale by some small commercial farmers, but the majority of aquaponics farmers are home hobbyists. Torcellini is a case in point. His fish tanks are scavenged food-grade polyethylene drums and to create his grow beds he "just welded some pipe together and filled them with a rubber pond liner".
If you search online there are plenty of kits available for those who want a ready-made option, but -- as Torcellini told me -- most backyard aquaponics farmers are very DIY. As if afraid I wouldn't be convinced by his jerry-rigged greenhouse, he then took me inside to show me his indoor system where he was using the fish poop from two of the biggest goldfish I'd ever seen to grow lettuce.