Why Has My Pond Gone Green?

Why has my pond gone green?



Green ‘pea-soup’ pondwater is due to large numbers of free-floating cells of algae floating in the water. The algae feed on the nutrients in the water and are a natural occurrence. Green water on its own is rarely harmful to fish or wildlife. Green water is not necessarily unhealthy, and clear water is not always healthy. Indeed, fish tend to have brighter colouration when kept in green water, as the pigments in the algae enter their diet and help boost the red colour in goldfish and koi.  www.watergardensolutions.co.uk

Green water is to be expected in new ponds as these generally have a higher level of dissolved nutrients. Nutrients are introduced in fresh tap water and in planting soil. Usually the green will fade of its own accord after a few weeks or months. The plants and microscopic pondlife establish, and a balance is achieved. Water problems are more common when fish are added to a new pond before this balance has been achieved. The green might return as a slight haze for a few weeks at the beginning of each season. (Green scums on the water surface and long strings of weed are a different type of algae - see our item on Blanketweed

Persistent Green

Green water can be a more persistent problem in shallow ponds; in those pools exposed to continuous sunlight; and in pools where nutrients wash in from surrounding soil. If your pond needs regular topping up with tap water, then nutrients in the tapwater (phosphates) will also encourage algae. It will also occur if larger fish or ducks are constantly stirring the pool base or disrupting the plants and balance of wildlife. Feeding too much fish food, or overstocking ponds with fish, will also increase problems with algae.

Check that green algae really are the problem by filling a jam jar with pond water and letting it settle. If the base of the jar ends up dirty grey/brown rather than green, then fine particles may be part of the problem. Raise the pump a bit off the pool base and/or consider an effective filter system.


Although green water can be treated with algicide and flocculating chemicals, these are short term solutions and can have side effects, especially if used without accompanying filtration. If green algae die back all at once, they can pollute the water and use up valuable oxygen, harming fish. Changing all the water will give only temporary relief, and the newly added minerals could encourage the pond to go green again in a short time.

Longer term controls

Establishing a good range water plants often helps. An efficient biological filter will usually clear the pool of algae at the same time as improving the water quality for fish. An ultra-violet (UV) unit will reliably control pea-soup algae without the need for any chemicals. Ideally UV units and biological filters are used together, though UV units may be used on their own in certain circumstances. ©

My UV unit has stopped controlling green water?

If a previously well-functioning UV unit stops controlling green water, despite water flowing through it at the correct rate, it usually indicates that either the internal sleeves have become dirty, and are stopping the light from penetrating the water, or the lamp needs to be changed. UV lamps normally require changing once per year, a job best carried out in the spring.

Contact Us if you wish to install a filter or UV unit or have an existing unit serviced.

See more Frequently Asked Questions

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