[Video removed because I couldn't figure out how to turn off the annoying autoplay. Go here to watch it.]
It works through a camera that uses recognition software that tracks objects based on their shape and color. The software then links each movement to different instruments that change in pitch and tempo as the fish patrol the tank. Fish that move toward the surface have a higher pitch. The faster they move, the faster the tempo.
The idea is to create audio aquariums for the blind. (via clusterflock)