Preventing algae in fish ponds and water features

One of the most common issues we get asked by our customers, whether in the shop or over the phone, is regarding excess algae, especially in spring and summer

Pond algae infestations vary in types but the common ones that usually affect ponds are the blue-green algae (which makes the
water ‘soupy green’) and the string algae or blanket weed.

It must be said, that a bit of algae in a fish pond is a very normal thing.  Algae is a natural part of any aquatic ecosystem, but it can sometimes grow too much and become unsightly and might be a sign of some sort of imbalance in the pond water chemistry or pond conditions.

Algae grow in any water body with exposure to sunlight and more so when there are nutrients in the water.  the more of either ingredient, the greater the algal growth will be.

There are a couple of factors that encourage the accelerated growth of algae in a pond such as:

Inadequate filtration:

Without an efficient filter, there is a high chance for blue-algae to take a foothold in the pond especially when such a pond receives lots of sunshine. To prevent this, we install the appropriately sized filtration system for your pond which will take into consideration several factors, including sun exposure, pond volume, and the types and size of the fish in the fish pond.  Generally, if your pond water is going green, you will most likely require an ultraviolet light pond filter which will sterilize the water and prevent algal growth.  You can read more about UV pond filters here and see what filters we have for sale here

We understand that some of this information is a bit complicated, so for further inquiry, you can give us a call and speak with one of our specialists that will help point you in the right direction.

Inadequate water flow leading to stagnation:

Imagine a river, when a river flows quickly it's usually clean and clear which is pretty to look at, but once that river slows down in the water flow, it no longer looks clean; it looks cloudy and if it completely stops flowing, then it can start going green.

Water flow is essential to prevent sediment from building up and inhibits beneficial aerobic bacteria to do their job effectively cleaning the water. This is why it’s important to have the right amount of water flow.  Each pond will require a certain flow in order to allow the filtration to work effectively.  You can read more about selecting the correct pump here

Speak to one of our specialists who will gladly help you out choosing the right pump for your pond.

You can see our range of filtration pumps here

Overfeeding fish

Fish are cold-blooded meaning their metabolism increases and decreases based on their surrounding temperature.

Therefore, in the winter months, fish require less feeding, and in the warmer periods a little more feeding.

When you apply excess food (even if the fish eats it), it builds up and adds excess nutrients such as Phosphate, Nitrite, and nitrate.

Because the fish are not making use of all of it at the time.  Fish should only be fed what they can totally consume in about a minute or so.  You can read more about it here


Phosphorus is one of those chemicals that can creep up within a fish pond or aquarium without you knowing it.  It doesn't seem to affect fish health, and if you have a good filter, the water will still be clear due to the UV light sterilization.

So why are phosphates a problem in the aquatic environment? Because string algae thrive off it! 

Algal growth within the pond is not only unsightly, but it can also lead to issues with your fish health.  Although algae put oxygen into the pond while it is photosynthesizing, it robs the water of oxygen during the night, so in heavily stocked fish ponds can lead to fish deaths.

Phosphates tend to accumulate in ponds because biological filtration systems do not deal with them at all, which means that over time, phosphate levels incrementally increase unabated. 

Sources of phosphates include fish food, leaves, detritus, and even tap water.  The better-quality fish foods tend to release fewer phosphates.

Come spring, when there are fluctuations in water temperature from warm to cool to warm and back to cool over an extended period of time can tilt the balance of filtration in a pond which can allow algae to take a proper foothold especially the Blanket weeds.


This chemical is broken down from organic compounds such as dead leaves, fish food, etc. A good way to prevent this is by adding lots of water plants into the pond. Plants help a lot to rid the pond water of excess nutrients, by making the nutrient inaccessible to the algae. At The Fish Works, we stock a wide range of water plants suited for ponds at different depths and sunlight tolerance.

Water plants not only reduce nutrient loads in fish ponds, but they will also block sunlight from hitting the water, directly competing with the algae for its ingredients to survive.  Speak to one of our specialists or better still, visit our store. You can see some of the types of water plants here

You can periodically always test for this chemical in your pond to keep you up to date with your pond's health. Buy your test kits here

Removing Phosphate

Removing phosphates from your fish pond can be as simple as doing water changes or using products that will lock up the phosphates and drop them out of the solution.  Doing so will make the phosphates unavailable for algal growth.  A great product to put into your pond periodically is Maintain for ponds which will bind the phosphates and add good bacteria to your pond to make it much more difficult for the algae to grow.

We recommend that all pond owners should check phosphate  levels monthly so that they can ensure that levels are close to zero at all times, especially if algae is an issue in your pond.

Lack of proper aeration leading to an increase in anaerobic bacteria activity:

Oxygen is essential for fish and the overall health of a pond, once anaerobic activity increases, a lot of things can go wrong such as infections, fish stress increase, excess nutrient build-up, and ultimately, algae take over!

A good way to control this is to keep the pond oxygenated. You can use an air pump to create bubbles.  But if you have adequate filtration, then this might not be necessary. Visit us or give us a call and we can help point you in the right direction.


There are many factors that influence algae growth be it string algae or blue-green algae. As you can see, there are a multitude of factors that contribute to algal growth, so give us a call or bring in a water sample and we can try to work out the root cause of your algal problem and treat the cause rather than just trying to treat the symptoms.  Pond owners need to discourage growth by making sure the pond is been filtrated properly with lots of water plants to take out excess nutrients and making sure dead leaves and other decay materials are removed in time.

Above all, do not overstock your pond with fish, and do not overfeed!