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Pond Algae Introduction and Links

Last Updated: 5/19/09

The Benefits of Algae
The Disadvantages of Algae
Quick List of Ways to Reduce Algae in Ponds
Aquatic Plants and Algae - pH and oxygen swings
Algae Links

The Benefits of Algae

1. All algae provides food for microorganisms which larger animals and fish can eat.
2. Surface and hair algae provide food for fish directly.
3. All algae absorb excess pond nutrients like ammonia, nitrate, and phosphate.
4. Suspended algae provides some shade to pond animals and plants in the spring.
5. All algae provide oxygen during the day (but use it at night).
6. Hair algae provides a soft spawning site for fish.

Limited amounts of algae are a good thing. A pond devoid of all algae is either sterile or being supported by outside forces (fish food feeders). Algae is a necessary part of a healthy, balanced pond. But, you can have too much of a good thing. Then, it becomes a bad thing.

According to Care2, "Algae, the first plants on earth, developed in the sea 3.5 million years ago and give off oxygen as they produce food, as other plants do. Today, algae produce over half of the oxygen that we breathe." Someone e-mailed me and said that in fact, algae developed 3.5 billion years ago! It does not matter; just know, it has been around a long time!

See the pond talk page for a discussion on algae benefits.

The Disadvantages of Algae

1. Suspended algae reduces clarity so that animals and plants cannot be seen in the pond.
2. All algae reduce oxygen levels at night.
3. All algae may cause pH fluctuations.
4. All algae may cause the death of submerged plants, water lilies, etc. due to either reduced light levels or strangulation in the case of hair algae.
5. Hair algae can clog filters, pumps, etc.
6. Finally, many people find algae ugly.

Quick List of Ways to Reduce Algae in Ponds

In no particular order:

1. Reduce the quantity of fish in the pond and/or the quantity of food given to the fish. Fish waste and fish food both yield algae food.

2. Increase the quantity of plants in the pond. Plants are nutrient sinks that compete with algae for light and nutrients. This could include a veggie filter (~10% of the surface area of the pond; half of the pond volume pumped through per hour); watercress in the waterfall or filter (see here); floaters like water lilies, water snowflakes, water hyacinth, water lettuce, salvinia, duckweed, azolla, etc. (see here); submerged plants like anacharis, cabomba, hornwort, foxtail, etc. (see here); marginal plants with their roots exposed to soil including fast growers like iris, cattails, and sweet flag; and more (see here).

3. Decrease the quantity of light into the pond. This could include using arbors, tarps, floating plants, lilies, or even moving the pond.

4. Use barley straw (see the barley straw page).

5. Use BZT Aquaculture (see my pond catalogs page, go here, or call them at 1-918-610-5205 to ask for the Ponder's Special (~$41 with shipping and handling).) or similar bacterial and enzyme concoctions like Microbe-Lift.

6. Add tadpoles and/or snails to consume debris and algae itself (see here).

7. Remove debris, dead plants and animals, dirt, etc. from the bottom of the pond.

8. Increase the size and/or efficiency of the filtration system. Clean out the filters on a regular schedule.

9. Add a UV sterilizer (see the UV sterilizer page).

10. Physically remove hair algae.

11. Be sure no chemicals run off the surrounding lawns and land into the pond. Fertilizers will directly feed algae. Pesticides will kill off the good insects and microorganisms in the pond which feed on algae. Herbicides will kill off plants and maybe some algae but once the higher plants are injured or dead, the algae will come back with a vengeance.

12. In ponds that are low in potassium, add potash (potassium-only fertilizer). This will help higher plants to grow if they are limited by a lack of potassium (K). If the higher plants grow, they compete better with the algae. Muriate of potash is 0-0-60 and is actually potassium chloride (KCl), sulfate of potash is 0-0-50, and magnesium sulfate of potash is 0-0-22. Put in about a tablespoon per 1000 gallons. Alternatively, buy Flora Fin by Tetra which contains potassium as well as other plant nutrients.

13. Check the pond's pH. If it is overly high (above 8), then finding a way to lower it may help reduce algae. In the long term, it is better to deal with the water chemistry that you are given then to try to adjust it all the time but short term, adding a pH-Down solution (H2SO4 or sulfuric acid) may help if the pond's pH is high.

14. Try adding cornmeal to ponds without fish. The corn gluten in the cornmeal suppresses plant and algae growth. If put into a pond with goldfish or koi, they eat it all so it cannot do its work (but you can try if you have fewer fish). This is an old algae remedy. Use one cup of cornmeal per 100 square feet.

15. Add bentonite clay to large or koi ponds. The clay acts as a coagulant and settles out algae. It will cloud the water in the short term and has other benefits for koi by providing minerals. One place that sells calcium bentonite clay is but most pond stores now carry it.

16. In parts of the world, there are ultrasonic devices that are supposed to kill algae. See LG Sonic for more on that and also Sonic Solutions which is in the USA.

17. Use an Algae Bar which slowly releases copper to kill algae. In the past, it was said adding copper pennies to a water body would reduce the algae as well. Note that in high enough doses (and under certain conditions), copper is toxic to fish. In lower doses, it may be toxic to invertebrates and other small animals.

18. Unless you wish a sterile environment that never stops fluctuating, do not use chemical algicides or herbicides. Only do so in swimming pools and sterile fountains. Even then, wild birds, etc. may drink it and get sick. Of the chemical algicides, I think AlgaeFix by Aquarium Pharmaceuticals is the safest. It should not harm fish or plants but is toxic to some invertebrates such as crustaceans. Most pond stores carry it.

19. Lorrie had this to share about filtering out pond algae on 6/20/07:
"What works just great for me in clearing up my big fish tank and pond is removing the foam filter that comes with your filter and replacing it with quilt batting. I cut thick pieces the size of the filter compartment and push it in. It will clear your pond in a matter of days - like a miracle! I have tried all the chemicals, and always feel horrible about it even though most say that these chemicals won't hurt the fish and all. This year we bought a bigger more powerful pump and filter for our pond, and our water has been so thick and murky, we couldn't see our fish (unless they swam to the surface). I remembered what a guy from our local pet store told us about using the quilt batting for our fish tank, and thought I would try it in the pond.... in 2 days the pond was CLEAR! It was amazing.
The quilt batting can be bought at Walmart., and it is cheap. It is white, and comes in a big round clear bag. All you do it roll it out and cut to the size you need. I usually cut a stack of pieces and then they are ready for next time. I put several layers in, in both sides of my pond filter. Check it in a few days and probably change the batting. It should be good for a couple weeks or more without changing again, depending on how bad the condition of your water is. Works like a charm!"

Aquatic Plants and Algae

Algae can smother plants and cause other problems like clogged filters and pH swings. Just like plants, algae grow using light and nutrients (mostly fish waste in a pond with fish). They use carbon dioxide and provide oxygen during the day. The process if reversed at night. Over night, a pond infested with algae may lose enough oxygen and/or have a pH drop far enough to harm fish.

Algae (and/or submerged plants) induced pH swings:

During the day, submerged plants and algae use carbon dioxide taken from the water where it is in the form of carbonic acid. Animals (fish, etc.) provide plenty of carbon dioxide. Thus, during the day, the pH increases or becomes less acidic. During the night, the submerged plants and algae release carbon dioxide back into the water where it re-forms carbonic acid which lowers the pH (becomes more acidic). This phenomenon is called the pH swing. Buffers in the water limit the pH change to within safe ranges for fish. Waters low in buffers like my pond may have more drastic shifts in pH. Adding a bag of crushed oyster shell to the filter can increase the water's buffering capacity (see the November 2001 newsletter for more information). You can also add various chemicals directly to the water but only if you are sure of what you are doing. I add baking soda (sodium bicarbonate) to my soft water pond to help buffer the water.

Algae (and/or submerged plants) induced oxygen swings:

During the day, submerged plants and algae release oxygen into the water which helps animals. At night, they remove some oxygen from the water. If they remove too much, the drop in oxygen concentration may cause some fish to gasp at the surface for air or even die. To prevent this, aeration must occur 24 hours a day in a pond. The water can be aerated by any combination of air stones, waterfalls, fountains, jets, or spitters.

More information:

For information on types of algae, preventing algae, algae eaters, and how to get rid of algae, go to my pond algae page. For information on dealing with algae in aquariums (algae eating fish, snails, chemicals, etc.), go to my aquarium algae page. For information on water chemistry, go to my water chemistry page.

Algae Links

These links were last checked on 4/18/06. Inoperable links were deleted or linked to from their archived versions if found on

Green Water: Myths, Facts, Theories II - article with some interesting theories on algae. The results of his findings would indicate that heterotrophic bacteria, which require room just as nitrifying bacteria, are vital in controlling algae in a pond. He believes that the bacteria produce some substance (enzyme?) which breaks down algae, and ponds with larger filters have more of these heterotrophs and thus clearer water. His conclusion is that "When algae dies and is subjected to aerobic bacterial decomposition by heterotroph bacteria, a by-product of this process is a substance, released into the water, that is toxic to the living algae."

Managing Nutrients to Control Algae - an article that says that the addition on potash (potassium) may help control algae (in certain ponds). This is also an archived version.

Pond Algae and Its Control - a nice summary of algae in ponds. This is also an archived version. [I am sensing a trend; algae sites do not stay around for long!]

A Pond Life Identification Kit - a UK-based web site with microscopic photos of various algae and other microorganisms as well as biological information.

Aquatic Ecosystems sells a lot of various algae controls and chemicals. For many other sources for algae-related supplies, see my pond catalogs page. Most every pond supplier sells at least a few algae treatments. The Pond Guy sells all sorts of algicides, and plant killers (which I would probably never want to use since most are so toxic).

United Tech - sell BZT, call them at 1-918-610-5205 to ask for the Ponder's Special (~$41 with shipping and handling). - Dr. Johnson's (of fame) site on algae.

What's that stuff in the water?

For links on barley straw, see the barley straw page.

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