The most effective way to treat and remove pond scum is to mechanically take it out of the pond with a net or pond rake. By removing the clumps of algae, as opposed to killing them letting them decompose in the pond, you prevent any re-release of nutrients that would otherwise fuel a future algae bloom. Simple dip nets and durable rakes with 3 to 5 inch teeth work well to pull out algae. It is also key to discard the detritus away from the pond. If left on the shoreline, the algae would decompose and rain would wash the nutrients right back into the pond.
Chelated copper is the most common active ingredient in algaecides available to consumers and lake managers alike because of its effectiveness in eliminating all varieties of pond scum. Typically found in liquid form, a water diluted mixture is made in a backpack sprayer or hand sprayer and then applied evenly over the surface of the pond scum. Copper-based chemicals must NEVER be used in the presence of any minnows, trout, or carp species, including koi and grass carp.
N on-Copper-Based Algaecides
For those ponds that do harbor the aforementioned fish, there are alternative chemicals that use a sodium carbonate peroxyhydrate active ingredient that is non-toxic to any aquatic life. Most readily available in a granular form, gloves and scatter box are used to evenly distribute the granules over top of the pond scum bloom. Similar to the fizzing action that occurs when baking soda comes in contact with water, so to will these types of algaecides bubble within 60 seconds of contact with pond scum.
Phosphorus is the primary nutrient pond scum needs to thrive- if it is unavailable the algae doesn't ever have the chance to form. Phosphorus removal products are used as preventatives to bind and render the nutrient unavailable. Aluminum sulfate is the most common form of phosphorus binding products.
Pond dyes are another preventative method that should be used before a bloom occurs. By coloring the pond water blue or black, less sunlight is allowed to penetrate the water column thus decreasing the ability for pond scum to carry out photosynthesis, or the primary method by which all plants generate energy.