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Algae and How to Control It

Adapted from several posted articles including the Water Garden News and the Smithsonian Botanical Institute via Pond Droppings 2/03

There are thousands of species of algae. Without them there would be no life on earth. “Algae are photosynthetic organisms that occur in most habitats, ranging from marine and freshwater to desert sands and from hot boiling springs to snow and ice. They vary from small, single-celled forms to complex multicellular forms, such as the giant kelps of the eastern Pacific that grow to more than 60 meters in length and form dense marine forests. Algae are found in the fossil record dating back to approximately 3 billion years in the Precambrian. They exhibit a wide range of reproductive strategies; from simple, asexual cell division to complex forms of sexual reproduction.

Algae are important as primary producers of organic matter at the base of the food chain. They also provide oxygen for other aquatic life. Algae may contribute to mass mortality of other organisms, in cases of algal blooms, but they also contribute to economic well being in the form of food, medicine and other products. In tropical regions, coralline algae can be as important as corals in the formation of reefs.”1

Trivia - Many of the white sand beaches in the Caribbean are mostly sun bleached and eroded calcium carbonate remains from green algae.

Algae grow quickly when water is warm, days are long (more sunlight) and organic material accumulates on the pond bottom. Warm water not only increases algae growth directly, but also increases fish metabolism that increases the organic load. Since southwest Florida has all the factors favoring algae growth, it can be a problem for pondkeepers. Let’s talk about the types of freshwater algae then move on to algae control.

Types of Algae: Planktonic algae thrive on the conditions listed above and will soon turn your pond water pea green making it difficult to see the fish.

These single celled organisms can multiply quickly and can die quickly. If they go through the bloom and die cycle, they can deplete your pond of oxygen and kill all the fish. (Trivia - There are over 8,000 species of green algae.)

Filamentous algae are single celled organisms that attach together to form puffy balls (like cotton balls), furry mats attached to rocks, or floating on the water surface. They can be mats or webs of nondescript green material. Some strands are long and gossamer like, drifting around your pond. It can be attractive until you run your hand through the strands and find the stringy threads clinging to your fingers.

Short velvet like algae that covers the bottom and everything else in the pond is beneficial. It uses nutrients from the water, provides oxygen during the day and the fish nibble on it. This type of algae cannot be totally eliminated with fish and plants in the pond.

Several species of filamentous algae are particularly adapted to growing on fast-moving water in waterfall areas. These species thrive in the bright sunlight (more energy for photosynthesis) and constant supply of nutrients suspended in the water flowing over the rocks. You can physically remove it if you like. Fish often eat the dislodged algae.

Blue-green algae (cyanobacteria) has received a lot of attention lately especially in Central Florida, due to health risks to humans. This has not been much of a problem in our area and is not usually a problem for pond keepers. (Trivia - Blue-green algae is really a bacteria because it lacks a nuclear membrane.)

Control: The green algae are not particularly bad for a pond, just esthetically undesirable. So, lets talk about algae control.

Mechanical Filters: One of the best actions you can do to maintain clear water is to make sure your mechanical filter is clean and working to capacity. Inspecting your filter more frequently during warmer weather helps avoid problems. In addition to your pump working properly, be certain that it is sized properly to your pond and fish population. Adding fish and fish growing are both factors that require more filtration capacity.

Biological filters: To increase the productivity of your filter system, add nitrifying bacteria. These products contain beneficial bacteria that jump-start your filter. It can take from a few weeks to several months to over a year for a biological filter to reach peak efficiency.

Besides filter systems, plants help control algae in several ways. Some plants compete for nutrients that could be used by algae. Some plants shade the water to reduce photosynthesis in the shaded area. You can experiment with a variety of plants to help prevent algae and enhance the beauty of your pond.

Underwater plants: Underwater plants are anchored to the pond bottom or planted in underwater pots. Almost any underwater plant can help add oxygen and shade some of the pond. Most underwater plants acquire their nutrition from the soil, so they don’t usually reduce the water nutrient load significantly.

Floating plants: Of all the types of plants, floating plants probably remove more nutrients from the water column. Their roots hang in the water with no soil required a al. Consequently they remove nitrogen and phosphorus directly from the water. In addition, they provide shade, thus reducing the light available for photosynthesis.

Floating leaf plants: The broad leaves of many floating leaf plats (lilies) are excellent shade producers. Some experts recommend that 50 to 60 % of your pond surface be shaded either by terrestrial plants (trees and shrubs) or by aquatic plants like lilies.

Marginal or emergent plants: These plants are also helpful in shading areas of your pond, especially the shallowest area prone to algae production.

Rainwater: Rain run-off flowing into your pond is a significant contributor to water nutrient load. (Trivia - 34% of the nitrogen load in Sarasota Bay is from rain.) In addition to nitrogen in the rainwater itself, water that flows over your lawn or planting beds can carry a high quantity of fertilizer. If this is happening in your pond, divert the runoff around the pond by either constructing a berm (mound) or swale (shallow trench).

Runoff with high pH: If the run-off flows over a concrete patio and then into the pond, it can also raise the pH of your pond water. This is especially true if the concrete are is less than one year old. Higher pH contributes to algae growth. A pH closer to neutral will decrease algae growth. Cement, limestone and marble will raise the pH of the water therefore contributing to algae growth. Invest in a pH testing kit.

Sludge: Another factor that makes algae control difficult is a lot of sludge (dirt and decaying organic debris) in the bottom of the pond. Bacteria products may help decompose this sludge but you should not expect them to totally get rid of large amounts on their own.

If you have an inch or so of sludge on the bottom of your pond remove as much of this as possible by hand (scooping, vacuuming, etc.).

Ultraviolet clarifier: If you cannot control the green water you may want to consider the addition of an ultraviolet sterilizer for your pond. An ultraviolet sterilizer will kill all of the algae that pass around the ultraviolet light. These units are sized according to your pump’s flow rate and the number of gallons in your pond.. The dead algae is then picked up by the filter to be washed away or broken down by bacteria if you use a biological filter.

Enzymes: Biological clarifiers (enzymes) help break down organic material (algae food) in the pond. Some commercially available products contain nitrifying bacteria as an added bonus. These products help prevent but are not an algaecide. Adding enzymes and bacteria help by building up the bottom of the food pyramid. This contributes to maintaining a balanced ecosystem in your pond.

No fish pond: If you have a pond without fish, you can use fountain chemicals to control the algae. One product that works very well and economically is Fountec. It is safe for plants, birds and other animals but it can NOT be used with fish.

Fish ponds without plants: You may have a pond with fish but no plants in which case you can use Pond Blocks. This is a slow release algaecide that will kill the algae (both single cell and multi cell algae). One block will last about a month and treat 250 gallons of pond water. Remember this is NOT safe for most plants.


* Reduce the organic load as much as possible. Fertilizers belong in the lawn (if at all), not in the pond.

* Keep sludge to a minimum . Organic matter on your pond bottom is algae food.

* Don’t over feed you fish. Many of them will graze on algae if they are not over fed.

* Don’t over stock your pond. One goldfish per 5 sq.ft. And one koi per 10 sq.ft. are a good rule for most ponds.

* Install an adequate size biological filter and give it time to work. This could take several months.

* Use many aquatic plants to shade your pond and reduce nutrient loads in the water.

* Use biological treatments (enzymes and bacteria) and give them time to work.

* The best pond is a balanced pond.

One of the most interesting and challenging aspects of koi and pond keeping is establishing and maintaining a balanced aquatic ecosystem. Since an ecosystem is a dynamic living group of organisms, our job is usually a matter of fine tuning or maintaining the balance, removing overgrown or unwanted plant material or adding water.

1 Smithsonian Institute: