Algae are diverse photosynthetic organisms, ranging from microscopic single-celled phytoplankton to large, multi-cellular seaweeds. While they form the base of many aquatic food webs, unchecked growth in human-controlled environments creates significant problems. In swimming pools, algae cause slick surfaces, cloud the water, and clog filtration systems, creating both a nuisance and a safety hazard. For ponds and lakes, massive algal blooms, often called “pea soup,” destroy aesthetic appeal and threaten the health of the entire aquatic ecosystem. The choice of algaecide must be precisely matched to the water body, as the specific environment dictates which chemical is safe and effective.
Primary Algae Killers for Large Water Bodies
The most recognized chemical agent for large-scale algae control in non-drinking water sources is copper. Copper-based compounds are broadly effective biocides that interfere with the algae’s cellular processes even at low concentrations. Copper sulfate is the oldest and least expensive option, frequently used in large lakes and reservoirs. However, it can precipitate rapidly, leading to accumulation in the sediment, which creates dangerously high copper concentrations toxic to sensitive aquatic life, such as trout and koi, a major concern for fisheries.
Chelated copper products are a safer alternative for large ponds and lakes. These formulations bind the copper ion to an organic molecule, allowing the active ingredient to remain suspended longer. This controlled release increases contact time with the algae, resulting in a more efficient kill with lower overall doses of copper, reducing the risk of sediment contamination.
In swimming pools and spas, the primary algaecides are the halogen compounds, chlorine and bromine, which also function as the main sanitizers. Chlorine, typically liquid sodium hypochlorite or stabilized tablets, is a powerful oxidizer. It is the preferred choice for outdoor pools because its action can be stabilized against ultraviolet light degradation using cyanuric acid. Bromine is often favored for hot tubs and indoor pools because it is more stable than chlorine in warmer water, maintaining effectiveness longer.
Specialty Biocides for Sensitive Environments
For aquatic environments containing fish, decorative plants, or other sensitive organisms, specialty biocides with lower toxicity profiles are necessary. Quaternary Ammonium Compounds, shortened to Quats, are non-metallic, surfactant-based algaecides used in pools and ornamental applications. These compounds function as microbial disinfectants by attaching to the algae cell wall and disrupting its structure.
Some polymeric Quats are engineered to be non-foaming and highly stable, offering long-term algae control when used preventatively in pools. Hydrogen peroxide has become a popular treatment in aquatic settings where high toxicity is unacceptable, such as residential ponds or aquariums. It is a strong oxidizer that breaks down rapidly into harmless water and oxygen, making it an environmentally friendly choice with minimal residual effects.
Hydrogen peroxide is commonly used for spot-treating specific patches of algae in aquariums without harming fish or plants, provided a proper low-concentration solution is used. In ornamental ponds, proprietary polymer-based algaecides are formulated specifically to be safe for fish and aquatic plants, including koi. These products often work by inhibiting algae growth or causing the algal cells to flocculate, or clump together, allowing removal by the filtration system.
How Algae-Killing Chemicals Work
Algae-killing chemicals achieve cell death through three primary mechanisms: oxidation, photosynthesis inhibition, and cell wall disruption.
Oxidation
Oxidizing agents, such as chlorine, bromine, and hydrogen peroxide, cause rapid cellular destruction. These chemicals release highly reactive molecules, like free oxygen radicals, that aggressively attack and break down the cell membranes and internal proteins of the algae. This process leads to the immediate rupture of the cell, effectively killing the organism.
Photosynthesis Inhibition
Photosynthesis inhibitors, most notably copper ions, interfere with the algae’s ability to produce energy. Copper ions bind to the enzyme systems within the cell that are necessary for the photosynthetic electron transport chain. By blocking the cell’s ability to convert light energy into chemical energy, the algae is starved and its growth is halted. This action is effective across a wide range of algal species, including the problematic cyanobacteria, often called blue-green algae.
Cell Wall Disruption
Cell wall disruptors, such as quaternary ammonium compounds (Quats), are cationic, meaning they carry a positive charge. Algae cell walls are negatively charged, causing the Quats to be strongly attracted to and bind with the cell surface. This binding action physically compromises the integrity of the cell membrane, allowing the cell contents to leak out and leading to cell death.
Safe Application and Non-Target Toxicity
Using algaecides safely requires strict adherence to manufacturer instructions to protect human health and aquatic life. Concentrated algaecides should always be handled while wearing appropriate personal protective equipment, including chemical-resistant gloves and eye protection. Accurately measuring the product is necessary, as overdosing can increase the risk of toxicity to non-target organisms.
The greatest risk to fish and other aquatic life often comes not from the chemical itself but from the rapid decay of a large mass of dead algae. As the biomass decomposes, the process consumes vast amounts of dissolved oxygen in the water, which can lead to a fish kill. To mitigate this, large bodies of water should be treated in sections, such as treating only one-third or one-half of the area at a time, allowing oxygen levels to stabilize between applications.
Copper-based products carry a specific risk of toxicity to invertebrates, such as zooplankton and certain fish species like trout, even at concentrations that kill algae. Therefore, users must be aware of the sensitivity of the species present in their water body before application. Furthermore, combining incompatible chemicals or applying algaecides simultaneously with beneficial bacteria treatments can neutralize the desired effect or cause unforeseen chemical reactions, so waiting periods are necessary.