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Two Types

1.       Suspended algae/green water/plankton/pea soup/single cell algae

2.       String algae/blanket weed/hair algae/filamentous algae.   But, the short velvet type of algae that covers everything in the pond is beneficial.  Provides natural appearance, uses nutrients from the water, provides oxygen during the day, fish nibble it. 


Causes of Excess Algae

1.       Algae needs nutrition [nitrogen, phosphorus] and sunlight.  The nutrition comes from bi-products from fish waste, dead and decaying leaves, decomposed fish food, and plants.  Excess debris on pond bottom can cause string algae.

2.       Temperature and season change:  air temperature warms in spring, beneficial bacteria are just starting to grow, plants not filled out yet, pond is full of nutrients from dead decaying leaves and debris so algae thrives.

3.       String algae tends to grow in water with high pH values.  Also tends to flourish in waterfalls and in shallow streams because sunlight is more intense there and water provides constant supply of nutrients. 

4.       Too many fish/Over feeding fish and excess food providing nutrients for algae

5.       String algae is concentrated around edges of pond because water temperature is warmer and there is less water circulation.



1.       Increase circulation to edges.  Pump at least ˝ the total volume of your pond every hour and have a filter that will filter the total volume of your pond every 3-4 hours.

2.       Manually clean edges with your hand or soft brush.

3.       Add barley straw bundles

4.       Create shade with 50% to 70% plant coverage over surface of pond.  Add plants of all aquatic types to compete with algae for nutrients.

a.      Water lilies’ spreading pads shade the water

b.      Underwater plants, like anacharis, [one bunch per one or two square feet of surface area,] and floating plants like water lettuce and water hyacinth with free roots absorb nutrients directly from water and help block light

5.       Because the algae are fed by ammonia (fish waste), be sure that you have a properly sized and biological filter working to capacity.  Be sure filter is clean.

6.       If pond muck on bottom is thicker than one inch, scoop, siphon or vacuum some out. 

7.       Introduce beneficial nitrifying bacteria which will starve out the algae. [E.g., Microbe-Lift.]  Bacteria also breaks down some sludge build-up.  Bacteria need place to grow:  biofilter, rocks, plants, filter media.  Be patient as pond ecology fixes itself as new bacteria grow.

8.       Beneficial bacteria’s effectiveness is enhanced by use of an aerator. 

9.       Balance the pH:  should be between 6.8 and 7.4.  Some experts, however, say not to be concerned about the pH of your pond.

10.   Prevent run-off from rain or other parts of yard, garden or patio into pond.

11.   In Koi pond use rock salt @1pound per 100 gallons as prevention, not treatment, for string algae.  Remove excess algae before dosing with this level of salt as the decaying vegetation will pollute the pond and reduce oxygen levels.  Don’t do this in a water garden as will harm the plants. 


What Not To Do

1.       Do not clean pond filter with tap water which will kill beneficial bacteria.

2.       Do not change the water or do major water change because will have green water again in a few weeks

3.       Do not use chemical algaecides because they may harm plants and the algae will return.  The dead algae builds up on the bottom of the pond and creates an organic “compost pile” to the pond.  This provides nutrients and will cause more algae to bloom in a few weeks.

4.       Ultraviolet clarifier will destroy floating algae but not recommended unless you have zero ammonia.  UV will not help with string algae.


Maintenance of the Organic Pond

1.       Remove dead leaves or spent flowers.  Use skimmer or manually remove.

2.       When plants begin to take over, remove some and share with club or friends, or compost.

3.       Leave an inch of organic muck on bottom of pond. 

4.       Use barley straw bundles as preventative from recurring string algae.  Since it takes 3-8 weeks for barley straw to become an active organic algaecide, start in early spring.

5.       Don’t over feed or keep more fish than your pond will support.  One goldfish per 5 square feet and one Koi per 10 square feet are a good rule for most ponds.  Or no more fish than can fit nose to tail across widest diameter of your pond.


The Big Hassle Makeover

1.       In spring remove fish and plants, water from pond.  Liberally sprinkle kosher salt [not table salt].  Let it sit for three days and then brush off and rinse and clean the pond thoroughly

2.       Then add barley straw bundles as preventative from recurring string algae. 


Some Elements of The Ideal Pond

1.       Bottom covered with submerged plants

2.       Skimmer and large biofilter

3.       No/few shallow edges.  Good water circulation.

4.       Pond is deep, at least 24”, to reduce sunlight reaching bottom.


J. Lewis 5/7/02 for the Tucson Watergardeners.  Sources: ; ; ; ; ; ;