Sunday 11 April 2021
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Getting Rid of Green Water in Your Fish Pond

Getting Rid of Green Water in Your Fish Pond

When people set out to buy a filter for their fish pond, they often confuse a UV clarifier with a UV steriliser. Indeed, ultra-violet rays are important for filtration as ultra-violet or UV light sanitises a pond or pool so it remains free of disease organisms and algae. If you don’t want your pond to turn an unseemly green colour, you need to make sure you use the right filter.


A UV Clarifier

Filters make use of UV clarifiers or UV sterilisers. While both types of mechanisms are used for filtering, they are slightly different in their design and purpose. A UV clarifier kills some bacteria in a pond along with the algae. The water flows faster through a UV clarifier than it does through a steriliser too.


A UV Steriliser

A steriliser kills 99% of the floating algae in a pond as well as other types of living organisms. The steriliser does not harm the beneficial bacteria that grows on rocks or in the filter but kills the bad organisms that float on top of the pond.


Because the water flows faster through a clarifier, many filtering systems make use of this part for disinfecting and filtering. For instance, the Oase pond filter FiltoClear is really great as it uses a UV clarifier to get rid of algae on a pond.

How a UV Clarifier Gets Rid of Algae

Known scientifically as chlorella vulgaris, single-celled algae need both nutrients and light in order to thrive and usually are not considered a risk to health in the wild. The growth of the algae, however, will prevent the visibility of fish in a backyard pond.


When algae cells pass through a filter that makes use of a UV clarifier, the ultra-violet radiation disrupts or kills the internal structure of the cells. In turn, the growth flocculates or clumps together and is broken down by the pond’s filter.

Most people that take care of a koi pond use a UV clarifier or sanitiser year-round. In some instances, pond owners may only use UV disinfection in the spring when there is an upswing in fish activity. In order to eradicate the green water that algae produces, you need to consider the stocking level of your pond as well as the flow rate. Part of the chemistry of a pond, algae growth is sometimes referred to as algae bloom. The pH of the water is not only impacted by this bloom but also by the presence of other plants. During the day, plants manufacture nutrients from the carbon dioxide that is dissolved in the pond as well as from the energy that is obtained through the process of photosynthesis.

If the algae bloom is significant, the water pH can fluctuate dramatically over a period of 24 hours. Although large fish can survive this kind of volatility, small fish are not as resilient. The pH, in effect, is a measure of the alkalinity or acidity of water with 7.0 considered neutral. Anything below that number represents acidity whilst any reading above 7.0 is an indicator of alkalinity. If the pH in a pond is over 7.01 and the ambient temperature increases, more ammonia will exist in the water’s chemistry, which can be toxic to fish. That is why UV, when used as a disinfectant, is an important contributor to filtering pond water.