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/Aquatic Gardens, Design, Construction & Maintenance

Algae and Their Control in Biological Ponds

By Bob Fenner

UV is of use in algae control

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner

After battling algae infestations for twenty years in the waterscape maintenance business, I learned to look carefully at systems that never had algae problems. Yes, there are such set-ups, and you can have one too.

Perhaps you'll be one of those fortunate people who make adequate provision in the design and construction of your water effect to preclude having "green water" problems. If not, or due to nutrient laden water and intense light you have "green soup" or you'd like an introduction to the whys and wherefores of algae and their control, read on.

The simplest oxygen-producing organisms on this planet are the algae. They are for the most part autotrophic (self-feeding), have no complex organizations and no sexual reproduction. They contain chlorophyll and other pigments, but have no true roots, stems or leaves.

Algae occur wherever there is sufficient light for photosynthesis, water and nutrients; In fresh and salt water, in soil, hot springs, snow, even on and in plants and animals. Along with some fungi, there are algae that live on bare rock as lichens in such forbidding areas as the Arctic. This is their world.

Algae Groups Include:

Blue Green Algae/Cyanobacteria

More closely related to bacteria than other algae they are often the scum on polluted, under-aerated/circulated, over-fertilized waters. They are typically bluish-black and slimy. Forms include single cells, clusters, threads and chains.

Green Algae

Are the most commonly encountered; they're found everywhere. Occur as floating, attached, swimming forms and seasonal surface blooms.

Brown and Red Algae

These are mostly marine; you probably know them as kelps, attached seashore forms.

Diatoms and Dinoflagellates

Are single celled, microscopic algae, ubiquitous, and mostly beneficial in terms of nutrient cycling, oxygen production, competition with undesirable forms. Though diatoms may appear as brownish scums, they rarely cause problems in ornamental water features.

Other Algae Groups:

Euglenoids, golden brown, yellow green algae and others that are generally not a problem in captive systems.

Overview of the Algae

Sometimes they're beneficial functionally and esthetically, and other times unwelcome guests, the algae are easily controlled if understood. Most can be avoided by designing and constructing your system to reduce light, and nutrient availability; algal proliferation, related problems can be lessened through regular maintenance.

In terms of long term cost, safety and ease of use, algae control methods can be divided into three categories on the basis of most to least appropriate; these are biological, mechanical and chemical controls. A few pertinent facts hold for all methods:


Chemical Activity:

Algae thrive in harder, alkaline water. It is advantageous to render the non-water part of their environment chemically inert. Rock and concrete should be treated to lessen reactivity with the water. When constructing, this may involve acid-washing to leach out alkalinity, use of plastic cements, foundation coatings.

For non-biological systems it is most appropriate to coat the basin/s with a sealing material. Some proper types are asphalt emulsions, chlorinated rubber paints and epoxies. Marine paints and others may be formulated to be toxic; read the labels carefully.

Rocks and other decorations should be checked for reactivity. They should not be detectable by smell or taste; or they may be checked by chemical analysis. This applies to all rock, including the waterfall, that comes in direct contact with the water. A simple assay involves breaking off a small piece of material, boiling it in water, allowing to cool and testing the cooled water in a container with inexpensive "test fish".


Most algae do better under stagnant conditions. Keep your water in motion with air pumps and/or pumps.

Light and Heat:

The more intense the light over the longer period of time, with the deepest penetration to the bottom, the more the algae will grow. Aquatic plants, circulation waves, shade from trees, walls, lathe, screen will help. Make your system's sides as steep and deep as possible and safe. Color the basin/s as dark as possible to reduce light reflection. Black is the best despite it's heat absorptive properties. During construction, dyes can be added to the concrete. The lower and more stable the temperature the better.


Please see the Pond Filters pieces for a more complete discussion of different forms of filtration. Particulate filters are least useful, but do aid by simply removing sediment that might provide space for algae growth. Chemical filtration is generally unrealistically expensive, but use of water-softening clays and carbons may go some distance in preventing full-on blooms if within your budget.

Biological filters 

Of several designs for systems with livestock, if properly engineered, are the key factor in keeping your system balanced in your favor. You can win by launching biological warfare with bacteria cultures purchased at a tropical fish store and having these microorganisms live in your filter, preventing algae growth by removing nutrient from the water.


Are useful in controlling algae. They cut down on light and use some of the nutrients otherwise available to algae. Water hyacinths, water lettuce, duckweed, alligator grass, lilies, oxygenating grasses are among many excellent choices.


Control of these is very important. Food for algae comes from feeding your livestock, fertilizing your plants. Be careful using fertilizers around your pond, a very small amount can produce several orders of magnitude weight in unwanted algae growth. Keep soil, toys, children, basically any and everything else out of the water.

Frequent, partial water changes are the order of the day for all kinds of aquatic gardens. They are the best way of diluting nutrients.

Control Methods:

So much for prevention; let's discuss ongoing problems:

Biological Controls:

Algae Eaters: Snails are the most widely used scavengers, but not always a good choice. Snails carry diseases for fishes as well as humans. Many are bisexual and hard to control population-wise, others die mysteriously, polluting the water.

Re-read about snails and check with your local aquatic garden regarding appropriate available species. Some fishes, like small koi, Dojos (Misgurnus anqullicaudata) are useful as biological controls.

Mechanical Controls: 

Second best to prevention and biological controls are manual methods of algae control. Routine brushing of the basin walls and vacuuming helps during partial water changes.

Two of my favorite all time tools for pond-keeping are a razor-blade equipped hula-hoe I saw first at the Disneyland Hotel in Anaheim's koi pond for giving string algae a "crew-cut", and PVC pipe notched at the end to catch, twist, turn and remove hair algae. Hmmm, I'm developing a strong desire for spaghetti!

Complete clean-outs acid and/or bleach washes are sometimes appropriate. Rock salt is a good abrasive to use in scrubbing biological ponds leaving some algae behind.

Another type of mechanical/physical control involves the use of ultraviolet sterilizers (which we'll cover separately) to chop up the DNA of free-floating algae, and protein skimming (aka foam fractionation) with or without ozone to remove algae "food". These high-tech. options are only mentioned here in passing as being too pricey and touchy for the majority of ponderers.

A good practice to get into is to keep plant material from touching, growing over the edge of your pond onto the water surface. This grants pest algae substrate for growth and reproduction. Keep those plants nipped back.

Chemical Means:

Using chemicals to control algae is the least desirable route in terms of cost, safety and long term effect. With most chemical algicides you can't have live plants.

There are several brands of chemical algae killers on the market, many of dubious value. The problem being they treat the symptoms only without dealing with the cause(s) of the algae problem, i.e. what are the factors that are contributing to this system being out of balance? Beyond this, all algicides are to some degree poisonous to other livestock; be careful.

If you do use algicides, keep a close eye on the dosage and be on the lookout for below acute toxic side effects.. Several products state that under "bad conditions" the dosage may be doubled or tripled. If your water starts foaming and your fish start gasping heavily at the surface, remove the fish or change a large part of the water.

An Integrated Management Approach:

Realistically, you will have to do what everyone else does; call on all the above mechanisms to balance the degree of cleanliness/lack of algae with the costs of maintenance. Controlling algae should take the form of:

1) Proper construction, filtration and water circulation.

2) Minimizing nutrient availability by under-feeding, preventing run-off and excess fertilizer from getting into the water.

3) Using your test kit to measure nitrates and keep them at an acceptably low level through desirable plant growth and water changes.

4) Using shading as necessary to cut down light and heating.

5) Manually removing algae and nutrients through vacuuming, netting and filter backwashing.

6) If absolutely necessary, stooping to the use of chemical controls; most preferably copper compounds.

A Conclusion:

After all this talk of controlling algae, it ought to be pointed out that sometimes it's better to "let it be". Algae growth is an indication of a normal, healthy state. Within moderation, algae help keep the pond balanced and stable. The trick lies in the word moderate. If you can keep the algae groomed, in one desired area or cropped to a short length on the walls of your system, this will be to your advantage. By having desirable forms, you can reduce the incidence of algae blooms.

Algae come in many shapes, colors and sizes. Many are beautiful, and often fish do well in algae-infested waters. If you want to control algae in your pond at least enough so you can see your fish, the above suggestions will help.

Bibliography/Further Reading

Blasiola, George C. 1991. Controlling algae in garden ponds; a review of preventive measures. Pet Age 3/91.

Ford, David. 1986. Why does my pond go green? FAMA 6/86.

Hanby, David S. Swamp filter... a revolution in pond keeping. FAMA 2/96.

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner
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