Some algae is essential for a healthy ecologically balanced pond....

Algae is not only a normal part of any pond but also a beneficial one. It is a food source for many aquatic animals, however it can also be the bane of nearly all pond owners.

Algae is the food source for tadpoles, so if you are trying to create a habitat for frogs, you need some algae.

Careful monitoring and management are the keys to most successful ponds, however there are ways to make life easier. A well designed pond can reduce the algae problems, by not creating an environment where the algae can flourish.

A basic understanding algae helps us avoid algae problems.

Algae problems are easier to manage if we understand, that like any plant, algae needs food and light to grow. By designing and constructing your water garden so that you reduce light and nutrient availability, combined with some regular maintenance, algae control is far easier to achieve.

What are some of the factors that increase the risk of algae problems?

  • High light levels. Algae like any plant, needs light to photosynthesise and grow.

  • High water temperatures stimulate faster growth.

  • Over stocking of fish, leads to more fish waste. More fish waste leads to higher nutrient levels in the pond water.
  • Excess nutrients is a major cause of algae problems.
  • Pollutants in the pond water can be the source of many problems for aquatic life. They may be chemicals or excess soil particles from the garden, others may come from the runoff water, that collects from gutters and drains before ending up in the pond. Some may become harmful or even toxic, leading to an unbalanced pond ecology. Some aquatic life may die, while others may run rampant due to the reduced competition.  
  • Poor water circulation. Most algae grow better in still water or stagnant pond conditions. 
  • High pH or alkaline water increases the risk of algal blooms. This is more common in ponds that are constructed from concrete, where the lime in the cement often leaches out. Care should also be taken in the careful use of certain types of rocks in your water features. For example, sand stone and some types of honey comb rocks may also increase the pH in the pond water.

Good pond design is the best way to prevent algae problems

Prevention is always better than the cure. Good pond design helps us to minimize algae problems and reduce the need for a pond algae treatment.

The 2 main factors that need to be addressed are light and food.


Light needs to penetrate the pond water for the algae to grow. Shade from trees, walls and especially other aquatic plants, all help to reduce the light entering the pond water and therefore the ability of the algae to grow.

Designing the pond side walls as steep and as deep as possible, helps to increase the pond volume relative to the pond’s surface area. This creates a more stable pond environment as the larger volume takes longer to warm up. Remembering that warmer water speeds up algae growth.

Colouring the inside of the pond as dark as possible also helps to reduce the light reflection.


The build up of excess leaves, that decompose in the bottom of the pond, can be the source of many nutrients. Regular pond maintenance, to remove the leaves, helps to minimise the excess nutrients.

Adding more aquatic plants. Aquatic plants consume the same nutrients as algae, so by increasing the amount of plants you effectively reduce the amount of food available for the algae.

Adding fish or other pond animals, increases the nutrient load due to the increase in their waste.

Overfeeding the fish or other aquatic life, leads to increased waste through uneaten food settling to the bottom of the pond and then decomposing to release more nutrients into the pond water. Only feed the fish what they can eat with in a minute or two. During Winter it is often best not to feed at all.

Over fertilizing the aquatic plants can increase algae growth, as they feed on the same nutrients as the plants.

Incorrect fertliser or fertiliser application. Slow release fertilizer should only be added to the plant’s root system. Slow release fertilizer tablets inserted into the soil are one of the safest methods of fertilizing pond plants. Liquid fertlisers should not be used.

Adding beneficial bacterial cultures helps consume nutrients and organic waste. These recyclers help to reduce the available nutrients and so reduce the potential algae growth.

Biological filters create an environment where the beneficial bacteria can grow and help to reduce the nutrients in the pond water. This is often required in fish ponds as the increased waste can lead to increased levels of toxic ammonia. The biological filters help to consume the waste and detoxicify the pond water.

Frequent, partial water changes help most water gardens. They are the best way of diluting nutrients.

What are the best algae control methods?

In order of importance we can classify algae control into 3 groups.

1. Biological algae control.

Biological algae control is essentially through competition. This may be through the addition of aquatic plants that compete with the algae for light and food. What would you rather have? More plants or more algae?

Beneficial bacterial cultures should be added to the pond water. Some of these cultures help to decompose the organic matter, such as leaves and fish waste, while others consume the nutrients that the algae feed on.

2. Mechanical algae removal.

Most particulate or screen filtration systems don’t really trap and remove algae very well. String algae removal is often difficult through the fact that it easily clogs up the screens and plumbing. These mechanical type filters do filter out sediment, which may be a food source for some algae.

Some blanket or string algae removal can be achieved manually, through scooping with your hands, or using various tools such as sieves , nets and screens to scoop them up. Sometimes a stick may be the simplest tool. Just twirl the stick around and you’ll find that the algae starts to wind up on to the stick.

3. Chemical pond algae treatment.

There are a vast number of pond algae treatment products available on the market. Some are Copper based and rely on the fact that algae is sensitive to the copper at relatively low concentrations. Care needs to be taken, as repeated doses or high concentrations can lead to other plants dying due to the increasing toxicity.

Other products contain chemical herbicides. These chemicals, like the copper based products, also affect the algae at relatively low concentrations. Similar precautions should be taken to avoid overuse and increasing toxicity.

Some chemical treatments do not actually harm the algae, but actually treat the pond conditions that may otherwise stimulate algae growth. In ponds with a high pH there are various chemicals that help to make the pond water more acidic. Activated Carbon products can also be added to the pond to absorb excess nutrients and so reduce the available food.

Balanced algae control

By introducing more plants into the pond you create more competition for both the available sunlight and also the amount of freely available nutrients in the water.

When you include fish into your pond you tend to increase the amount of biomass (organic waste) and hence the amount of nutrients increases.

In most small ponds the plants cannot be the only source of control. Filtration systems should be included in the pond design. Biological filters help to control the nutrients in the pond and UV light filters can also be introduced to assist in the control of green water algae.

Through the monitoring of pH, carbonate hard water, ammonia toxicity, high levels of nitrates and nitrites we can determine the best course of action to help create a more stable pond environment which then reduces the risk of algal outbreaks.

Frequent, partial water changes between 10 and 25% will help most ponds. They are the best way of diluting nutrients and toxins in the pond water.