Crystal Blue Copper Sulfate is a great pond algae control product for filamentous or string algae. Copper sulfate should be used as a pond algae treatment on actively growing algae in water temperature above 60F. When getting rid of algae in a pond, it is always important to treat 1/2 of the pond algae at a time to avoid killing too much algae and depleting the water of oxygen. Copper sulfate is safe for most varieties of fish (see restrictions below) when used properly. Granular copper can be applied in one of three different ways: broadcast crystals, place crystals in a burlap bag and drag behind boat, or dissolve crystals and spray. The application method we recommend is to dissolve the crystals in hot water and spray actively growing algae.