Virginia, is it algae that is fouling the water? Some folks swear by barley straw as a natural alternative to chemicals:
http://ohioline.osu.edu/a-fact/0012.html http://www.bestfish.com/barlybal.html http://www.exoticwaterscapes.com/barley-straw.php?gclid=CPrSydjMhZMCFQh7IgodUwe6BQ -- Renee M. in Michigan
