They are used as phytoplankton control in some sewage ponds and livestock settling ponds. They are also used some in the catfish industry to control blue-green algea, the one that causes off flavor.

http://www.sciencemag.org/cgi/conten.../292/5525/2250

http://www.sciencemag.org/cgi/conten...i;292/5515/203