You can raise tropical fish in Minnesota. But why would you want to? To feed the lettuce, of course. It’s called aquaponics. The veggies filter the water, and the fish fertilize the veggies. Lots of people are trying to figure out how to make it work on a commercial scale. In Minnesota, researchers at an indoor fish and vegetable farm say the future is bright for small aquaponics operations. Very small. Like, in the back yard. BURN's Chris Julin filed this story for Marketplace.