Anyone else ever seen / heard of this form of growing plants?

I went to a family friends house, and on the back porch, the man had built a contraption with plants and goldfish. he had 3 fish (koi, i think) in a plastic tub, with a few tubes attached to the tub, and it came up to the part where the plants (mainly vegetables) were (they weren't in dirt either, it was a rocky material) and distributed water, then went back down to the fish tank. He wasn't there to explain it to me, but what does this do, clean the water and use it over again? If so, how expensive would it be do build and maintain this type of thing?