Pond scum, often an unsightly blanket on the water’s surface, is a general term for various fast-growing aquatic organisms. This includes filamentous algae, which appear as long, hair-like strands, or planktonic algae, which cause the water to look like pea soup. This growth is a natural response to an imbalance, typically an overabundance of nutrients in the water. For owners of small, recreational, or ornamental ponds, restoring balance without resorting to harsh chemicals is the goal, requiring a multi-step approach that combines immediate physical cleanup with long-term biological and nutrient control methods.
Immediate Physical Removal Methods
The physical removal of existing material immediately improves the pond’s appearance and reduces the overall organic load. Filamentous algae, often called string algae, is best extracted by hand or with a long-handled brush or rake. The technique involves twirling the algae strands around the tool, much like winding spaghetti, to pull out large masses.
This manual extraction is important because it removes the biomass that would otherwise decompose, thereby preventing the release of stored nutrients back into the water. For surface mats, a fine-mesh pond net can be used to scoop and skim the floating material. After removing the bulk of the scum, a partial water change can help dilute the concentration of suspended algae and dissolved nutrients.
Removing approximately 10 to 20% of the pond water and replacing it with fresh, dechlorinated water is a common practice for routine maintenance. These changes help lower the levels of nitrates and phosphates, which are the primary food source for algae. However, avoid large, sudden water changes, as they can shock fish and destabilize the pond’s beneficial microbial ecosystem.
Controlling the Nutrient Source
Algae blooms are a symptom of a deeper issue: an excess of nitrogen and phosphorus, which act as fertilizer for the scum. Addressing this root cause is the most effective long-term strategy for maintaining a clear pond. These nutrients accumulate from various sources, including fish waste, decaying plants, and external runoff.
One of the most significant sources of nutrient input is fish food, especially when overfed. Only feed fish what they can consume completely within about five minutes, as uneaten food quickly breaks down into nutrient-rich waste. Similarly, reducing the total fish population can lessen the biological load and the amount of waste produced, directly lowering the nutrient concentration in the water.
Decaying organic matter, such as fallen leaves or dead plant material, settles at the bottom, forming a nutrient-rich sludge called muck. This sludge slowly releases phosphorus and nitrogen back into the water, fueling continuous algae growth. Regularly using a pond vacuum or net to remove this accumulated debris prevents this slow-release fertilization. It is also important to manage the pond’s perimeter by ensuring that lawn fertilizers or pesticide runoff cannot enter the water, as these external sources are highly concentrated with nutrients.
Utilizing Aquatic Plants for Competition and Shade
Introducing or increasing the population of aquatic plants is a powerful biological method to outcompete algae for the necessary resources. Plants actively absorb the nitrogen and phosphorus from the water, effectively starving the algae. They also provide shade, which limits the sunlight penetration required for algae photosynthesis.
Surface plants, such as water lilies, water hyacinths, and water lettuce, are particularly effective at providing shade. These floating plants have large leaves that cover the water’s surface, aiming for a coverage of 50 to 70% to significantly reduce the light available to submerged algae. This shading mechanism also helps keep the water temperature cooler, which further inhibits rapid algae proliferation.
Submerged plants, or oxygenators like hornwort and anacharis, grow entirely underwater and are exceptional at nutrient absorption. They directly compete with algae for dissolved nutrients while simultaneously releasing oxygen into the water, which benefits fish and beneficial bacteria. Marginal plants, such as cattails and irises, are planted in the shallow edges and act as a natural filtration system. If plant coverage is initially insufficient, non-toxic pond dye can be added to the water to mimic the shading effect by blocking light penetration.
Harnessing Natural Decomposers and Additives
Commercial formulations of beneficial bacteria are comprised of living microorganisms that consume excess nitrogen and phosphorus. These bacteria help digest the organic sludge at the pond bottom, turning waste into harmless byproducts like carbon dioxide and water.
By accelerating the decomposition of muck, these bacterial additives reduce the nutrient reservoir that fuels algae blooms. The effectiveness of these microorganisms is significantly increased when the pond is well-aerated, as most beneficial strains are aerobic and require oxygen to thrive and rapidly process the organic material.
Another widely used natural additive is barley straw, which works as an algistat, meaning it inhibits the growth of new algae rather than killing existing cells. As the straw decomposes in the presence of oxygen and sunlight, it is thought to release compounds, possibly hydrogen peroxide, which prevent new algae from forming. For best results, barley straw should be bundled in netting, positioned near the surface where oxygen levels are high, and applied early in the spring before algae growth begins.