Filamentous algae, commonly known as string algae or blanket weed, is a pervasive problem for water feature owners. This organism forms long, hair-like strands that quickly weave into dense, unsightly mats across the water surface or submerged structures. The overgrowth is not merely an aesthetic issue; it can choke pumps and filters, interfere with recreational use, and reduce dissolved oxygen levels when the dense masses begin to die and decompose. Effectively eliminating this nuisance requires a comprehensive strategy that moves beyond simple, temporary removal to address the underlying environmental causes.
Identifying Filamentous Algae and its Causes
Filamentous algae is easily identified by its texture and appearance, which is distinct from the microscopic algae that causes green water. The algae forms thread-like colonies of cells that feel slimy or cottony when handled, often appearing bright green, though they may turn yellow or brown as they decay. A simple stick test—if you can lift the algae out of the water in a cohesive, stringy mass, it is filamentous algae.
The proliferation of these mats results from two primary environmental factors: excess nutrients and sufficient sunlight. Excess nutrients, particularly phosphorus (phosphate) and nitrogen (nitrate), act as fertilizer for the algae, allowing rapid growth. These nutrients often enter the water from sources like:
- Decaying leaves
- Grass clippings
- Fish waste
- Runoff from nearby fertilized lawns
Algae also thrive in shallow areas where sunlight easily penetrates, fueling photosynthesis.
Immediate Manual Removal Techniques
While addressing the root cause is necessary for long-term control, immediate physical removal provides quick, temporary relief and is a fundamental first step. The best technique for direct removal is to manually “twirl” the algae onto a tool, much like spaghetti on a fork. A specialized algae brush on a telescopic pole, a pond rake, or even a clean stick can be used for this process.
This method allows large quantities of algae to be extracted in a single action. It is important to remove all harvested algae completely and dispose of it far from the water’s edge. Allowing the removed material to decay nearby will cause the nutrients it contains to leach back into the water, fueling a new outbreak. Manual removal extracts the problem and the nutrients it has consumed, offering a temporary reduction in the overall nutrient load.
Chemical and Biological Treatment Options
Chemical and biological treatments offer two different pathways for controlling existing algae and reducing the conditions that support its growth. Targeted chemical algaecides provide a rapid kill, with products typically falling into two categories: copper-based and peroxide-based. Copper sulfate is an effective algaecide, but its use requires careful calculation based on water alkalinity, as it can be toxic to fish such as koi and trout, especially in soft water.
Chemical Treatments
Peroxide-based treatments, such as sodium percarbonate, work by releasing oxygen to oxidize and damage the algae’s cell structure. These treatments are fast-acting, breaking down into water and oxygen with no long-term chemical residue, making them a gentler alternative for water features with sensitive fish. Always adhere strictly to the manufacturer’s dosage instructions and treat no more than half of the water body at a time to prevent a sudden die-off that could deplete dissolved oxygen and cause a fish kill.
Biological Control
Biological control focuses on starving the algae rather than killing it. This involves introducing beneficial bacteria and enzymes, often sold as sludge reducers or pond cleaners. These aerobic microorganisms actively consume excess organic matter and nutrients (sludge, nitrate, and phosphate) in the water column and sediment.
By outcompeting the algae for their food source, these microbial treatments help restore the natural balance of the aquatic ecosystem. They work best when water temperatures are consistently above 50°F and when paired with sufficient aeration to support the oxygen-dependent bacterial populations.
Environmental Control for Recurrence
Achieving lasting freedom from filamentous algae depends on modifying the aquatic environment to make it inhospitable for future growth. The most effective long-term strategy is robust nutrient reduction, which starves the algae at its source. This involves regularly skimming the water for debris, minimizing fish feeding to prevent excess waste, and ensuring that nutrient-rich runoff from surrounding landscapes is diverted away from the water body.
Chemical tools like phosphate binders are also highly effective, working by chemically reacting with free-floating phosphates to transform them into an insoluble compound that sinks harmlessly to the bottom. Once bound, the phosphate is no longer available as a food source for the algae, even at levels as low as 0.025 parts per million. These binders are best applied after any major algae removal or chemical treatment to lock up the nutrients released by the decaying material.
The use of aquatic plants, such as water lilies and floating plants, provides natural light reduction by covering up to 50–70% of the water surface. This shading limits the sunlight needed for algae photosynthesis, while the plants simultaneously absorb nutrients, directly competing with the algae. Installing a proper aeration system, such as a bottom-diffused aerator, increases dissolved oxygen, which promotes the growth of sludge-digesting bacteria and helps bind phosphorus in the bottom sediment.