Clearing Up Green Water

by Elmer Epistola

Posted: September 3, 2004

If left on its own,  even a pond  with crystal clear water will turn into a pea-green soup in a matter of days, making all the koi in the pond invisible, except perhaps during feeding time.  Even then, all that can probably be seen are the heads of the koi bobbing up and down the water surface as they grab one pellet after another.  It is impossible to enjoy your koi under green water.

Green water is caused by a population explosion of algae in the pond.  Algae are microscopic, single-celled plants, which can easily multiply to a density of as much as 7,000-15,000 per milliliter.  With that many of them in the water, it's no wonder that the water turns green.

The ideal conditions for algae to grow rapidly in number are: 1) warm and well-lighted water; and 2) an abundance of nutrients.  The key to 'clearing up' green water, therefore, is to prevent the algae from multiplying by removing the ideal conditions for their growth.  

We all know that the warmth of the water and the availability of light for photosynthesis is modulated by sunlight, which is why algae flourish much better in summer.  Shading the pond is a big boost to eradicating green water because it diminishes sunlight, which is necessary for algae growth.  Unfortunately, shading a pond can be difficult and costly, and can even ruin the elegant beauty of the natural surroundings of your pond if built from improper construction materials. 

Using trees as shade likewise has some drawbacks, one of which is the mess created by its leaves falling onto the pond.  Lastly, koi do need some sunlight during the day, so shading the pond completely might not be a good idea.

Constrained as we are in limiting the amount of sunlight reaching the pond, we can still decrease the nutrients in the pond to hamper algae growth.   It's a matter of knowing where the nutrients come from and curtailing these supplies as well.

Koi excrete ammonia into the pond.  Certain bacteria convert these ammonia into nitrites, which are further converted by other bacteria into nitrates.  Ammonia and nitrites are extremely harmful, and even deadly,  to koi.  Nitrates are less harmful than these two, but they are powerful fertilizers for all plants in the pond, including algae.  Coupled with warm water, a suitable pH, carbonate hardness, and the presence of other nutrients like phosphates, nitrates are a sure-fire way of multiplying algae in large numbers.

Figure 1.  Pond nuisances such as green water (left) and blanketweed (right) are caused by algae

Aside from koi wastes, nutrients for plants in the pond come from the decaying organic matter in the pond.  Plant and animal matter that fall into the pond such as leaves, twigs, bird droppings, and the like are common sources of these decaying matter.  Uneaten food will likewise decay in the pond, and eventually contribute to the nitrate level of the pond.

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