Algae can play a vital part in the health of your pond. The right algae (in the right amounts) can introduce fresh oxygen to your pond, provide food for your fish, and add a touch of beautiful liveliness and color to your pond’s aesthetic. 

But algae is a living organism that is very sensitive to its environmental conditions. It’s less likely that your pond will kill off any algae you have growing (your fish and other plants will probably show signs of sickness before then). The bigger concern is algal bloom when different factors in your pond environment encourage the rapid and intense growth of algae. 

When there is too much algae in your pond, it can pose several problems as the algae spreads to take up nutrients and space that are vital to the health of your fish and plants. In this way, blooms can throw off your pond water’s nutrient balance in significant ways that will affect the health of your pond and its inhabitants.


There are several types of algae and, in general, they serve a positive function in the ecosystem of your pond. 

The ecosystem of your pond includes every living thing which is taking part – everything from the smallest, simplest bacteria to the koi you’ve had for over a decade. The pond’s inhabitants form a cohesive system which supports and protects itself. Every part plays an important role in the healthy functioning of the pond, which means that any part out of balance can spell disaster for the rest of the ecosystem. 

While algae can serve as a food source for small animals like bait fish or fingerlings, their more general purpose in a pond ecosystem is as a photosynthesizing organism. Like the other plants in your pond, during the day algae consume carbon dioxide in the water and produces oxygen, a necessary gas for the fish. 

This might lead you to think that more algae is better. But there are three reasons that isn’t the case: 

First, algal overgrowth tends to mean unattractive and murky, green-ish water.

Second, while algae may produce that vital oxygen during the day, or sunlight hours, it’s important to remember that algae and other pond plants consume oxygen after sundown. Too much algae could mean oxygen depletion in the hours before sunlight, when the algae begins producing oxygen again, putting your fish in danger. 

But the real issues with algae come during the die off which follows algal growth (promoted by sunlight and warm temperature, among other factors). During die offs, which can be caused by lack of sunlight, chemical treatments, or sudden cold, the dead algae sink to the bottom of the pond to decompose, giving off large amounts of nutrients and gases which can suddenly upset the balance of your pond water.  

Healthy algae growth relies on the presence of the right natural bacteria, a good nutrient balance in the water, and an adequate filtration system that can manage waste produced by the fish and plants.

Algae is affected by several factors:


Waste comes from a few sources. It can be waste products produced by the fish and other animals in the pond, or it may come from plant matter that is no longer alive. External sources around your pond like other animals, plants, or debris can also collect in the pond and sink to the bottom to join the other organic materials produced by animals and plants in your pond, breaking down over time and affecting algae growth.  

The sludgey layer at the bottom of your pond is a combination of different organic materials which have floated to the bottom and settled, decomposing over time. This sludge gives off nutrients and gases which promote algae growth, so it’s important to keep a regular eye on this part of your pond maintenance. 

Deep cleans which involve draining and pressure cleaning are helpful in maintaining the correct algae levels in your pond. 

Still water will also trap “bad gases” which promote algal growth. Still water also makes it difficult for new, fresh oxygen to replenish the pond’s stores as the fish and plants use it. 

A water fall or aerator is an easy and cost-effective way to agitate the water so that unwanted gases are released from the water and so that fresh oxygen can be introduced.

Nutrient overload which promotes algal growth can also happen during winter when fish and plants are less metabolically active, and therefore use up less nutrients in the water than they normally would in the warmer months, resulting in an overload. 


Water that has been inadequately filtered or treated will result not only in the waste and nutrient overload described above but will also deplete the natural bacteria which help cycle nutrients through your pond system. 

Because of algae’s sensitivity, you can see why it’s crucial to use the appropriate filtration systems and chemical treatments to maintain the quality of your pond water. Additionally, there are some simple maintenance steps you can take to help keep algae levels under control in your pond. 


Leaves and twigs that find their way into your pond can eventually sink to the bottom and slowly release nutrients which encourage algae bloom and growth. 

Use a pond skimmer to remove these leafy intruders, and if possible, try not to build ponds near overhanging plants (and vice versa, avoid planting near ponds if you’ll have to deal with lots of leaf and/or fruit drop).


Concrete has a high pH level, which is an environment that can encourage algae growth. A better option is to construct your pond with gravel lining the bottom and with clay for the sides. These materials have a lower pH level.


Algae growth is promoted by sunlight. 

Cover the surface of your pond with floating plants like water lilies, hyacinths, and water lettuce. The flowers (and the leaves that they float upon) will absorb sunlight and shade the water below, keeping the algae from getting too much sun exposure.

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