Great New York Times Article:

As its license plates proclaim, Minnesota is the Land of 10,000 Lakes.
Now a Minneapolis-area company says it has figured out the secret to the state’s famously crystalline watering holes: moss.
Specifically, species of Sphagnum moss that the start-up, Creative Water Solutions, envisions will keep tens of thousands of swimming pools clean while drastically reducing the use of chlorine and other harsh chemicals.