Pond Algae and Green water. How can I get a crystal clear pond?

Green water and pond algae problems such as blanketweed and string algae are annoying. The most common problem that ponds experience surprisingly does not relate the to fish, but to plants, – and unwanted ones at that, such as green water and blanketweed.

You can take a little comfort in knowing that you are not alone if your pond becomes plagued with unwanted plant growth. Ponds are usually afflicted with either green water or blanket weed (and sometimes both) at sometime throughout the season. If you take early action, you can reduce the impact that unwanted algae, particularly green water, will have on your pond.

Green water, – is it really all that bad?

Green water offends the eye, creating a green blot on our aquatic landscape. It is regarded as the water gardening equivalent to leaving your garden to become over-run with nettles, dandelions and goose grass. It makes your wonderful creation look neglected and second-rate, reflecting badly on your abilities as a pond keeper. But take some solace in the fact that green water offers many benefits to your fish and pond life promoting improved health and colour. In fact, commercial breeders and farmers of ornamental pond fish such as goldfish and koi positively encourage green water to flourish in their ponds, recognising that fish benefit from living in such conditions.

Don’t be lulled into a false sense of security:

Most ponds enjoy an unprecedented degree of clarity over the cold winter months. Such clarity makes it possible to see down to the bottom of even the deepest ponds and is a factor of the cold water temperatures and very short day lengths. Under these conditions, algae growth is retarded, and any algae that survived through last year is no longer able to thrive. However, as soon as the day lengths start to increase and the sun’s rays get hotter, pond water temperatures will also rise, creating ideal conditions for the primitive microscopic plant life that create green water.

The accumulation of nutrients in the water over winter (particularly nitrates and phosphates) fuels the growth of these remarkable, opportunistic plants. Because of their size and mode of reproduction, algae can soon multiply out of control, blooming from a few dormant cells or cysts into a pond of green soup, being the first plants to capitalise on these favourable conditions, dominating the pond’s biology and appearance.

The transformation of smaller ponds is fastest, as these warm up a lot quicker than the larger, deeper ponds. The microscopic single celled algae become so dense that the clarity is soon lost, in a similar way to the density of water droplets can transform a fine mist into a dense fog.

Recognising that green water is an increasing probability at this time of year, and that we want to keep our ponds as clear as possible so we can see our fish (that’s what we bought them for!), what can we do to prevent it from taking hold? Learning from tackling weed problems in the garden, if possible we need to find an equivalent to the methods of weed control, such as ground covering mulches, competitive planting or selective weed killers.

What control methods are available to us?

There are a multitude of different approaches for solving green water problems, each with their own benefits (and costs) and with the added complication of whatever comes into contact with the algae, can also affect other pond life, including our desirable pond plants and fish. This is particularly true when considering chemical treatments.

Methods available for the stillwater pond


A significant number of pond keepers who have tried many different methods of controlling green water (and blanket weed) have found that shading the pond is very effective. Erecting a pergola, and stretching green house shading across the top can cut out a great deal of direct sunlight that causes algae to thrive. As it does not involve treating the water, the fish and most aquatic plants will continue to thrive. Shading can also be achieved within the pond by adding natural vegetable dyes to the pond water, cutting out the sunlight. They change the appearance of the pond water to dark blue, but offer a long term, effective solution to algae problems.

Chemical treatments.

These include algicides that actively kill algae and blanket weed. These chemicals must be used correctly and accurately because overdosing can lead to desirable pond plants being affected. If your pond is largely free of plants, this does not present a problem.

Competitive Planting

The first of 3 greener or ‘natural’ remedies involves using desirable pond plants to out-compete the algae for sunlight and nutrients. A pond can take several years to become fully ‘balanced’ with desirable pond plants eventually winning the battle. This is the reason why natural, mature water bodies are rarely plagued with algae.

Barley Straw.

A second natural method of controlling green water is to add barley straw to the pond. The natural decomposition of barley straw causes the slow release of compounds that lead to the pond water becoming mildly toxic to primitive plants. This method can take several months to work as its effectiveness relies on the bacterial breakdown of the straw.


A more recent solution to the green water problem is to add a supply of beneficial bacteria that breakdown the nutrients on which the algae thrive. With regular, weekly dosing, these bacteria will continue to digest and utilise the nutrients that would otherwise have fuelled algae growth.

For the recirculating pond.


The final solution to green water problems is guaranteed to work by most manufacturers and is the only method of those discussed that requires a pond pump for it to work. The unit is called a Uvc (Ultra-Violet clarifier), and uses ultra violet light to control green water. A pond pump is required to pump the green pond water through the Uvc (which is positioned outside the pond), where it clumps together and is removed by the pond’s filter. The running costs are minute compared to other treatments and as long as the pond is fitted with the correct sized UV, it will be completely effective against green water. Its bulb will need changing every 12 months, with springtime being the best time to install a new one, with the subsequent 4-5 months presenting it with its hardest work.

Depending on your pond’s circumstances, you have a range of alternative methods for controlling green water to choose from. Even if you get caught out this spring with green water, a guaranteed method of controlling green water is available in a Uvc. However, never succumb to the temptation of substituting your green pond water with tap water as this nutrient-rich water will take you back to square-one, turning green again in a matter of weeks.

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