Nikhil Arora and Alejandro Velez didn't know each other when they enrolled in the same business ethics class at UC Berkeley. After their professor, Alan Ross, mentioned during a lecture that mushrooms can grow in coffee grounds, they approached him separately to learn more. Ross didn't have much to add about fungi, but he did introduce the curious students to each other.

"We turned Alex's fraternity into a test kitchen," Arora says, "and couldn't believe that one of the buckets actually started growing oyster mushrooms." After much research—including carting the 'shrooms to legendary Chez Panisse and asking chef-owner Alice Waters to taste them ("It was a good thing we didn't know who she was," Arora says)—they had a grow-it-yourself mushroom kit ready for market.

Since launching their company, Back to the Roots, in 2009, they've sold 350,000 of the $20 kits, diverting 3.5 million pounds of refuse from landfills per year. Now they're expanding into a new realm: home-size aquaponics.

After six months of "head-down execution mode," they designed a three-gallon plastic tank that grows lettuce, basil, and wheatgrass out the top and houses fish downstairs. It's called the AquaFarm ($60), and its elegant package says simply: "Fish feed the plants. Plants clean the water."

To fund its launch, Arora and Velez went on Kickstarter, rustling up $250,000 from 4,097 supporters whose feedback helped them tweak design elements.

In the AquaFarm's self-contained ecosystem, just one betta fish's ammonia-rich poop supplies enough nourishment for five plants. The package includes seeds, fish food, recycled-glass rocks, and a Petco coupon for a 99-cent fighting fish. It's sold in more than 1,000 stores, including Petco, Nordstrom, and Whole Foods.