The following are some tried-and-true methods that will not only help you treat algae, but also help prevent it. However, note that these solutions will not permanently eliminate algae. For year round solutions to algae growth and complete algae elimination, check algae elimination.

Natural Cover and shade

If your pond gets too much sun, create shade. Natural cover can be provided by surface-growing plants such as duckweed and other plants like water lilies and water hyacinths. Duckweed is very effective toward reducing algae in the lagoon. In wastewater lagoon, duckweed must be kept from the effluent by inserting surface baffles in front of the effluent weir. In aerated ponds and lagoons, it is not necessary to periodically harvest the duckweed, nor does the duckweed appear to result in significant accumulations in the bottom of the lagoon. Floating grids placed across the lagoon surface have been used to ensure surface coverage. However, several aerated lagoons covered with duckweed have operated successfully without grids. Duckweeds is a temporary solution that only partially block sunlight so it should not be intended for long term use.

Floating plants, such as lilies and lotus, provide shade and reduce direct sunlight in the pond to control the growth of algae. Add submerged plants that release oxygen to the water, such as anacharis, hornwort and parrot’s feather. As a guide, one bunch of six to seven strands of oxygenating plant can be added to every two square feet of water surface, and submerged by tying to a rock or planting in a soil container.

All aquatic plants also absorb nutrients and starve the algae. After initial plant introduction, green water may occur, but will last only a short time. Established marginal plants can be planted around the periphery of the pond or in shallow sections of the pond. These are also effective in absorbing nutrients and providing shade.

One popular way to introduce plant life into the pond system without putting plants into the main pond is to construct a plant filter. A plant filter is a simple channel or small filtration pond through which water from the pond is fed at a relatively slow rate. Fast-growing plants (efficient nutrient removers) are grown within this small pond in planting baskets or are free-floating, such as water lettuce or water hyacinth. The plant filter should ideally be lined with about 2” of pea gravel, which is the best substrate to root the plants. The pea gravel catches debris and acts as a bed for beneficial bacteria. As these plants grow, they absorb nutrients from the water and “out-compete” algae to control its growth. Generally, the plant filter needs to be stocked with plants equaling approximately one-fifth the surface area of the main pond.

Other more effective options include floating wetland and floating islands such as the BioFlot from Advanced Water Treatment Technologies. These will provide instant shade, help cool the water, and will absorb nutrients. Those covers also offer additional benefits such as evaporation reduction.


Algae thrives in poorly oxygenated water. Your pond will get an immediate boost if you install a waterfall, fountain, or aerator to agitate the pond surface and help release gases such as CO2. This also enables the water to absorb more oxygen, which will, in short time, help reduce the presence of algae.


    As was discussed above, if a lagoon basin treating a domestic wastewater is fitted with mechanical surface aerators that provide a power intensity of at least 6 W/m3 of basin volume (30 hp/106 gal), the turbidity of suspended solids is sufficient to minimize algal growth. At lower mixing intensities, algae will grow providing the HRT is sufficient. However, all lagoon basins, including those that are used for sedimentation (polishing), should be mixed a level of about 1 W/m3 of basin volume (5 hp/106 gal). Such mixing is beneficial from several points of view. Without mixing thermo stratification will occur, thereby permitting the retention of undisturbed surface layers for relatively long periods of time. Such conditions provide an excellent environment for algae to become established and grow.

Mixing will also exhaust the carbon dioxide from the system. For wastewaters, such as those from domestic origin in which there is an excess of nitrogen and phosphorus, carbon dioxide can be growth limiting during a portion of the diurnal cycle. During the night hours when light is not available, carbon dioxide accumulates as the result of respiration of the microorganisms in the lagoon. At dawn, when light does become available, the rate of consumption of carbon dioxide through photosynthesis exceeds that of respiration and, as a result, the store of carbon dioxide is depleted and algal growth becomes limited. In other words, the carbon dioxide accumulated during the night hours is stored for use in the daytime hours. Carbon dioxide concentrations as high as 25 mg/L have been observed at night in lagoons (Williford and Middlebrooks 1967). Since at sea level the saturation concentration of carbon dioxide is only about 0.42 mg/L at 20° C, mixing by aeration will remove significant quantities of carbon dioxide from the system during the night hours, thus ensuring that carbon dioxide becomes growth limiting earlier in the day. During the day, when carbon dioxide is growth limiting, aeration does not significantly replace carbon dioxide in the system because the concentration gradient is too low. As will be discussed in later notes, aeration in settling basin is a must, not only because of the mixing that is created, but also, for the maintenance of dissolved oxygen in the water column. Such maintenance reduces feed back of CBOD and nitrogen from the benthal deposits.

Copper Sulfate

Copper sulfate has long been used by waterworks personnel to control algal growth in reservoirs. Some waterworks personnel use a standard dose of 1 mg/L of copper sulfate which is sufficient to kill most types of algae. However, care must be taken to protect fish in the receiving stream. Trout, which appear to be the most sensitive of the fish, do not tolerate copper sulfate in concentrations greater than about 0.14 mg/L (Steel and McGhee 1979). It has been reported that the combination of chlorination and copper sulfate has given excellent results (Courchene et al. 1975).


Several studies have shown that chlorination will kill algae. The focus of most of these studies has been on the impact that algae have on the chlorine demand of plant effluents. In these studies, the chlorine doses used have been large (5-20 mg/L) and the contact periods short (15 min to 2h), conditions under which algae are killed and lyse. At least two authoritative studies, however, have shown that much lower chlorine doses (2-4 mg/L) over much longer contact periods (>10h) will impair algal growth (Echelberger et al. 1971; Kott 1971). This suggests that by continually adding chlorine in a relatively low dose in a aerated lagoon settling basin, effluent algae reduction would occur as a result of a lower growth rate.

Intermittent Discharge

    Algae respond to the diurnal variation in light by moving vertically through the water column. King et al. (1970) found that during the afternoon hours, the particulate COD at 8 inches below the surface of a facultative lagoon was about four times that at the same depth during the night hours. Such vertical migration suggests that effluent quality might be improved if the daily flow is released only during the night, or from two different depths over the diurnal cycle.

Phosphate Removers

Phosphate originates from many sources. It is present in the food you feed your fish. It is often added to tap water by municipalities. And, it can come from fertilizers in run-off water. If your test indicates high levels of phosphate, use phosphate-removing media in your filter to remove both organic and inorganic phosphate.


     As photosynthetic organisms, algae require light to grow. Per unit volume of lagoon basin, the quantity of light energy available for such growth is proportional to the surface area. For a basin with vertical sides, an increase in the depth will decrease the surface area proportionally. However, because of the trapezoidal cross section typical of lagoon basins, an increase in depth does not always decrease the surface area. Figure 1 illustrates the relationship between the two variables for a basin with a volume of 2840 m3 (750,000 gal) and with side slopes of 1 (vertical) 3 (horizontal). For such a basin, an increase in depth will decrease the surface area up to a depth of about 3 or 4m. Beyond which depths the surface area begins to increase.

Figure 1. Surface area vs. depth for a lagoon basin with a volume of 2840 m3 and side slopes of 1(vertical):3(horizontal)


  Lagoon depths of 3 or 4 m will also create a more favorable geometry for mixing with surface aerators. Reduced surface areas will position the mixing zones in closer proximity.

Hydraulic Retention Time (HRT)

    Retention time is the most influential factor controlling algal growth. In a lagoon basin with a depth of at least 3 m and fitted with mechanical surface aerators that provide a power intensity of about 1.6 W/m3 (8 hp/106 gal of basin volume) or less, algal growth can be expected to occur if the HRT exceeds about 2 d (Fleckseder and Malina 1970; Toms et al. 1975). If, however, the lagoon basin is divided into two or three cells in series by curtain walls, algal growth can be expected to occur only if the total HRT exceeds about 3 d, and 3.6 d. respectively (Rich 1999). Thus, the post fitting of a lagoon basin with curtain walls may reduce effluent algae. At greater aeration power intensities, shading provided by the suspension of settable solids reduce algal growth. At an intensity of 6 W/m3 (30 hp/106 gal), very few algae will grow.

Natural Barley

Barley straw is effective in controlling only algae. It is not effective against higher plants, or vascular plants, such as duckweed, watermilfoil, or pondweed. Available in many forms – mats, pads, pellets, sticks, bagged straw, and even liquid extract – barley releases soluble carbohydrates that cause algae to clump together and sink. The mats also provide both biological and mechanical filtration, and, to keep your pond attractive, can be cut to fit in virtually any filtration system where they will be out of sight. Barley Straw is not considered a pesticide by the U.S. Environmental Protection Agency.

The usual recommended quantity to use is 225 pounds (or 4–5 bales) per surface acre of water. Reported recommended rates range from as low as 110 pounds per acre, and higher rates are recommended for water that has a high percentage of suspended sediment or appears muddy. But it has been recommended that a rate of 450 pounds per acre never be exceeded to avoid the decaying straw from depleting the water of oxygen.

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Water Soluble Dyes

Certain non-toxic, organic water soluble dyes that blocks out the specific light rays utilized in photosynthesis are used for killing algae. Some of these dyes, which leave the water a natural teal blue, have been used to kill algae in sewage lagoons. Such dyes are marketed commercially.

Adding Salt

Salt can help control algae blooms. There is a drawback with using salt to combat algae in your pond though and that relates to the fact that high enough salinity will also harm or kill your aquatic plants.

For example, common plants like water hyacinth and lotus will begin to die back at 0.10% whereas water lily won’t die off until 0.5% and to deal with algae effectively you will want to shoot for 0.25 to 0.30%. You will have to determine if salt makes sense for your algae problems based on your resident species of aquatic plants.

A good way to determine the right amount of salt is to the use an online calculator and once added a simple way to check your salinity is with a commonly used tool called a refractometer.