I once saw a documantary where, i think,  see horses where used for the same reason. They where held in a  small aquarium where a gentle stream of water was flowing on top of. If the water was good the would swim at the top, near the stream and if the water was bad they would swim at the bottem.<br>Edit: I didn't find the exact video I watched but I found a video from a Swiss research organization who are using water fleas and freshwater shrimps to check processed water from wastewater facilities for its quality.