Dealing with Green Pond WaterThere are few things more disappointing for the water gardener than to see the water of your lovingly built pond turn soupy green. In the past, dealing with the curse of green water could often turn into a major headache, but today, combining some of the old, time-honoured remedies with a spot of modern technology can usually be relied on to sort the problem out, once and for all.

What is green pond water caused by?

Green water is caused by the growth of large numbers of single-celled algae, which live suspended in the water and is fuelled by light and excess nutrients in the water – typically making its appearance as the days begin to get sunnier in the spring.

Although it is most commonly encountered in recently constructed ponds, where the water chemistry and planting has not yet settled down, older, more established water features are not immune.

The principal culprit is nitrate, a nutrient which forms naturally as left-over fish food, fish faeces and dead plant material decays. Nitrate test kits are widely available and very simple to use, but often simply clearing out fallen leaves, cutting and discarding dead water-plant foliage and avoiding over-feeding your fish can make a big difference.

Natural control methods for green pond water

A variety of methods can help to deal with green water. Natural methods include avoiding fertilising the pond itself – nor the area around it – removing decaying matter, not over-stocking with fish and sensible planting.

Surface plants compete with algae for light and so having as much as a half or two-thirds of your pond covered with vegetation can make a big difference, while trees or bushes can also be used to give shade, further helping to reduce the levels of sunlight reaching the water. However, these approaches may not always be enough in themselves, as green water may still happen in early spring, before the protective plants have developed new foliage.

For another natural additive, barley straw could fit the bill. Administered either as pouches of straw, or doses of straw extract, it is an approach which some have hailed as a miracle cure – though not all pond-keepers are unanimous in this opinion. It takes a month or two to show any effect, but the natural enzymes are said to have remarkable abilities to inhibit algal growth – making it effective against blanket weed also.

Chemical treatments for green pond water

Chemical treatments are also available to deal with the problem. Although they can be expensive and do not provide lasting protection, they can be very effective, particularly for newly constructed ponds. They must be chosen carefully if the pond has already been planted or stocked with fish and if you do decide to go down this route, then reading the label carefully and using the product in accordance with the instruction is obviously critical.

Perhaps the single greatest advance in recent years in the battle against green water has been the development of reliable and affordable ultra-violet (UV) clarifiers. Aside of its effectiveness in destroying the algae which cause the problem, this approach has a number of other advantages. It has no effect on water quality and because it takes place away from the pond itself, it poses no threat to the plants or fish. Easy to install and cheap to run, a good quality UV unit combined with a biological filter system and an appropriate planting regime offers the best possible means of dealing with the green water scourge – though it is important to remember to change the UV bulbs every year. A number of manufacturers produce UV units to suit a range of pond sizes, widely available from garden centres and other retail outlets.

Left unchecked, green water algae can turn the most beautiful of water features into a pitiful sight in a very short time, but the good news is that with a little careful thought and a bit of practical effort, it should be fairly straightforward to keep your pond crystal clear.

A primer for Pond Water, and why it is important.

Pond water serves as a habitat not just to fish but many microorganisms that we can’t even see with our naked eye, yet they play an important role in keeping the water quality at its best. The quality of water is judged by several parameters which help in understanding how contaminated it is for aquatic life. These parameters include the oxygen level, carbon dioxide  level, salinity and temperature of the pond water.

One might question why it is important to measure these parameters, a pond is an ecosystem which will only survive when these factors are at an optimum level for the aquatic life to survive in, which includes plants around a pond as well.

How do I measure the quality of the pond water?

Out of all these parameters, the level of dissolved oxygen is the most crucial as the feeding patterns and immunity of fishes depend on it. A dissolved oxygen meter (DO meter) is used to measure the levels of oxygen, normal level being 4 ppm (parts per million). Low oxygen levels occur if there are algal blooms crashing in the pond water, this happens due to the consumption of high oxygen levels during the decomposition of algae.

Low oxygenated pond water and dangerous gasses

When levels of oxygen are dangerously low, levels of harmful gasses such as carbon dioxide increase. CO2 makes it difficult for the fish to take oxygen, causing death. Temperature is another important parameter which affects feeding of the aquatic animals, and the levels of oxygen consumption in water. Although fish change their temperature according to the external temperature in order to survive, they cannot tolerate abrupt temperature changes. These sudden changes occur when fishes are put into a pond from an aquarium without letting them first adapt progressively to the temperature. To regulate the temperature, aerators are used which can help in controlling the pond water temperature when transferring the fishes. These parameters are linked to each other which is helpful and makes the process easier to understand for people who are new at maintaining a pond.

Detecting problems from green pond water

Regularly checking the parameters will allow you to understand how all these parameters are related along with the feeding patterns of fishes. Growth and health of these fish will be easier to monitor, along with the health of the plants nearby the pond. More than one parameter is mentioned since a single parameter might not help you see the whole picture but the way these variables interact help to judge the quality of pond water. The sooner you detect the problem, treatment with oxygen pumps, aerators and even aquatic plants could be used to minimise the damage and bring the parameters back to normal.

Green pond water FAQ

Is green pond water healthy?

The presence of green water in a pond does not always mean that the pond is unhealthy. Algae, a single-celled plant organism, are the primary cause of green pond water. This is often a result of high levels of nitrates and phosphates in the water, which create an imbalance in the pond’s ecosystem. However, algae are not always harmful to a pond’s ecosystem, and an overreaction can cause more harm than good.

How do I get rid of green water in my pond fast?

A UV clarifier and filter are a quick and effective way to remove green water from your pond. This method involves a pond pump that circulates the water past a UV light, which kills the algae causing the green coloration. The filter removes the dead algae from the water, and you can see noticeable improvements in water quality within a short time.

How can I stop my pond water going green?

To prevent pond water from turning green, you must address the root cause of the problem, which is the excess nutrients in the water that algae thrive on. Minimising waste and debris by reducing overfeeding and keeping the pond clean can help. Adding aquatic plants that compete with algae for nutrients and beneficial bacteria that break down waste and reduce nutrient levels in the water can also be effective. Additionally, regular water testing can identify and help correct nutrient imbalances.

What causes green pond water?

Green pond water is caused by an overgrowth of microscopic algae, single-celled plant organisms that thrive in nutrient-rich environments. Excessive nutrients such as nitrates and phosphates in pond water encourage the growth of algae, leading to green water. Factors such as warm weather and prolonged sunlight exposure can also contribute to algae growth in ponds

What is the best treatment for green pond water?

A UV clarifier is a recommended treatment for green pond water. It uses a UV light to kill the algae cells in the water, leading to a clearer pond. The amount of time it takes for a UV clarifier to work varies depending on several factors, including the size of the pond, the strength of the UV clarifier, and the severity of the algae bloom. With a properly sized UV clarifier, green pond water can clear up in a week or less.

How can I clear my green pond water fast?

Using a UV clarifier is the quickest way to clear green pond water. The UV light kills the algae cells in the water as it passes through the unit, usually leading to clearer water within a few days. However, it’s crucial to note that the UV clarifier only treats the symptom of green pond water and not the root cause, which is often an excess of nutrients in the pond. To prevent future outbreaks, addressing the underlying cause of the issue is essential.

What causes a pond to turn green?

An abundance of microscopic algae is what causes pond water to turn green. Algae are naturally occurring single-celled plant organisms in ponds that play a crucial role in the pond’s ecosystem. When the balance of nutrients in the water, such as nitrates and phosphates, is disrupted, algae blooms can occur. These blooms result in the pond water turning green. While algae are not necessarily harmful, excessive blooms can cause low oxygen levels and harm aquatic life.

What kills green algae in a pond?

While algaecides can be used to eliminate green algae in a pond, they should be a last resort due to their potential harm to other aquatic life. Copper-based algaecides are commonly used and are sprayed over the pond to kill the algae. However, before using an algaecide, other methods like removing debris, reducing overfeeding, and introducing beneficial bacteria should be tried first. Prevention is also critical, and regular maintenance can prevent excessive algae growth. Maintaining the right pH level, using a UV clarifier, introducing aquatic plants that compete with algae, and reducing the amount of sunlight that reaches the water are effective prevention methods.

How do I clean my pond water naturally?

There are several ways to clean pond water naturally. Aeration and agitation of the water help keep the pond clean. Using a pond rake to remove debris, adding floating, marginal, and oxygenator plants to reduce nutrient levels, and introducing colorants to reduce sunlight that reaches the water and prevent algae growth are effective. Beneficial bacteria can also be introduced to the pond water to break down organic matter and reduce nutrient levels. Maintaining the correct pH level and regular water changes are also important.

Is green algae bad for a pond?

Green algae in a pond is not inherently harmful, but excessive growth can cause issues like discoloured water, scum at the pond edges, and dense mats of growth under the surface. Algae can spread rapidly under certain conditions and harm aquatic life by reducing oxygen levels. Additionally, excessive growth can lead to imbalanced nutrient levels in the pond and further water quality problems. Preventing excessive algae growth by regularly maintaining the pond’s ecosystem is crucial. This can include reducing overfeeding, removing debris, introducing beneficial bacteria, and using a UV clarifier to maintain a balanced nutrient level in the water.

How do I clear my green pond water?

A pond filter, pump, and UV clarifier are effective in clearing green pond water. The pump circulates the water through the UV clarifier, where the ultraviolet light damages and disrupts the algae. The size of the pond determines the size of the equipment required for optimal performance. Regular maintenance, such as removing debris, reducing overfeeding, and using beneficial bacteria, can prevent future outbreaks of green pond water.

Should I remove green algae from my pond?

Algae are crucial members of a healthy pond ecosystem, providing food for species at the lower end of the food chain. Rather than removing it, controlling its excessive growth is a better approach. Regular maintenance, such as removing debris, reducing overfeeding, using beneficial bacteria, and introducing aquatic plants that compete with algae, can prevent excessive algae growth. This approach can help maintain a healthy balance in the pond’s ecosystem.

Last Modified: April 5, 2023