Green Pond Water

How to Treat and Prevent Green Pond Water

How to treat and prevent green pond water is one of the most common maintenance related questions we get here at It is also one of the greatest sources of stress for many new pond keepers. So how do you get rid of it? It is completely attainable to achieve crystal clear water by following a few important suggestions when designing and maintaining your pond.

What is Green Pond Water?

Green water is caused by the accumulation of millions of single cell, microscopic spores of green algae suspended in the water. Single cell algae have the ability to make your crystal clear pond water into what looks like pea soup with limited visibility. Algae that causes green water is not to be confused with string algae (also called blanket weed), which looks like a stringy weed that bunches together and often clogs filters and drains.

What Causes it?

The biggest stimulant to green pond water is the sun. Although some sunlight is healthy to your pond (just like it is to people), excessive exposure will no doubt cause problems. An 'algae bloom' is when single cell green algae reproduces at a fast rate, fueled by both sunlight and nutrients in the water. There are other things that can contribute to green water as well that are more related to basic pond maintenance: things like an improperly operating biological filtration system, too many fish for the size of the pond, or high levels of nutrients that promote algae growth. The main nutrient of concern is nitrogen, which happens to be a byproduct of fish waste and decaying organic material in a pond. To read more about basic pond maintenance, please see our page on water garden care.

How to Get Rid of it

The easiest way to 'starve' an algae bloom is to reduce the amount of sunlight exposure to the pond. There are a couple ways to achieve this. The most obvious way is to build your pond in a place where it receives some shade in the day. Shade will not only help limit photosynthesis in Algae, but it will also help keep your pond water cooler in the summer, which brings with it a host of other benefits. Another option for introducing some shade is by using floating water plants. By covering 30 to 70% of the surface of your pond with floating plants, you will essentially be getting much needed shade, along with the added benefits that plants provide a pond. These benefits include reduced nitrate levels, which helps control string algae/blanket weed. To read more about the benefits of pond plants, please see our page on plants for ponds.

ultraviolet clarifier
An Ultraviolet (UV) Clarifier

Ultraviolet Clarifiers

A UV clarifier, sometimes called a UV filter, is a device that emits ultraviolet radiation at a specific wavelength that damages single celled algae on a cellular level. After passing through the clarifier, the algae then clumps together at which point it can be trapped inside a skimmer or other mechanical filter. Please note that a UV clarifier only really affects green algae that causes green pond water. It has little or no effect on other common problems such as string algae, parasites or bad bacteria.

Some people think of a UV filter as a 'band-aid' approach to fixing green pond water. In some ways they are correct, IF there is an underlying problem with the design or maintenance of your pond that is obviously stimulating algae growth (like excessive sun exposure). However, sometimes a completely healthy pond might go through a period of algae bloom. In such cases, a UV clarifier can really help clean out the green algae in the water, as well as prevent future outbreaks. Most koi keepers like to keep their pond water as clear as possible for the viewing of their prized fish. In such cases, a UV clarifier can really help 'polish' the water for crystal clear viewing.

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