Some form of this is quite common. My town's water treatment plant uses fish. They have all of their automatic monitoring things, but they also have a school of

Some form of this is quite common. My town's water treatment plant uses fish. They have all of their automatic monitoring things, but they also have a school of fish in the water coming out of the last stage of the plant, just before releasing it. They keep an eye on the fish. If they die, they know there's a problem and shut things down. On a tour, I asked the obvious question: isn't that a little late? By the time the fish are dead, haven't you released a large quantity of hazardous water? The response was basically a shrug and "it's better than nothing". I think I like the Polish approach better.