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Green Water and Slimy Algae

Green water is a problem that many pond keepers have to deal with, especially in the summer months. Algae can form fur-like growths on logs, rocks, and on the sides of the liner. These can feel slimy, cottony, or coarse.

19 August 2021

There are a number of contributing factors that cause the single cell algae to thrive in a pond and this is what creates green water. It is therefore recommended to consider the cause before choosing a treatment. If your pond is suffering from blanketweed then our blanketweed blog may be of interest to you. 

New Pond

If you have a new pond, or have recently performed a water change, then it is worth knowing that it does take time for your ponds water to naturally find its balance. Every time a water change is performed it does take time for the water to resettle, so unless you have tested your pondwater and the results have indicated a water change is required, then it is best to avoid doing this. You can read more about this on our water quality blog.

Green water is not necessarily an indication of bad water quality, only a test can indicate your water balance. If you need to add water to your pond, then rain water is the preferable method. Tap water includes a number of different chemicals, and although treatments can take out harmful chemicals (chlorine and chloramine), there is a theory that the other remaining chemicals can encourage algae to grow.  

Nutrient Imbalance

Excess nutrients in your pond water will feed the algae, so it is important to limit them. Sludge that forms at the bottom of your pond from rotting organic matter, fish waste and fish food is a main culprit for adding excess nutrients. Limiting these will help decrease your green water. Removing the sludge from the bottom of your pond with a net, treatment or a pond vacuum will help, but if sludge really is a problem then you should look at why it is forming. Can you trim/prune pond plants before they fall into the pond? Are you over feeding your fish? Is your pond overstocked with too many fish? More information on sludge is available on our Sludge blog.

It is also worth considering if nutrients are being added to your pond indirectly, for example over use of pond plant fertilizers, rain water run-off from garden fertilizers or the use of non-aquatic planting soil in your pond plant baskets (aquatic soil has been purposely created for this reason).  


Having extra aeration into your pond will provide a massive boost to the resilience of the ecosystem. Aeration helps to boost the healthy bacteria in your pond which helps to keep the ecosystem in check and reduce algae development. Please see our blog posts on this. In short, adding extra air into the pond helps to control various chemicals in the water (notably Ammonia, Nitrites and Nitrates). This then helps to prevent algae blooms as the algae will have fewer nutrients and its reproductivity severely hampered.


Aerating a pond can be done very easily and stylishly; from adding a waterfall or a fountain or adding plants. Plants are one of the best ways to preven algae; as the plants use the nutrients in the pond for growth, algae will therefore have fewer nutrients. This competition stops the algae from spreading and leaves your pond looking stunning, with a healthy mix of plantlife. Floating plants are also really good at providing shade (which reduces the temperature of the water and provides a hiding place from predators for your fish!).

Overfeeding Fish

Regardless of how many fish you have in your pond, overfeed fish is a regular problem. It's nice when visitors feed your fish for the thrill of seeing them eat; but leftover food can sink and decompose, which affects the chemicals if your pond. You should feed your fish only when you can see that there is no food left and its best to feed fish in smaller, more frequent meals; it's easier to do this with a feeding ring as the food remains in one location.

Number of Fish

An overstocked pond will more fish waste and more uneaten fish food. It is common for fish to breed and grow in a healthy pond, resulting in overstocked ponds with filter systems unable to keep up. You may have the correct size pump and filter for the size of your pond, but if your pond is overstocked or the fish breed rapidly, then the equipment will not be able to keep up. What may have been suitable when first installing your pond, may no longer be suitable.

The general guide for fish levels in ponds is below:

  • 3kg of Golfish per 1000 litres of pond water e.g. 4000 litre pond will support up to 12kgs of Goldfish
  • 1kg of Koi per 1000 litres of pond water e.g. a 4000 litre pond will support up to 4kgs of Koi  

Fish are obviously quite difficult to weigh for the average pond keeper! You can use the notion that 1” of fish is equal to 30g. This is just an estimation, but it will help you gauge an idea.

Filtration Systems and UVC's

In addition to the above, you may also find that you have the wrong filtration unit for your pond.

With Koi, we recommend filtering your pond once an hour, where as with goldfish, once every two hours. If you are unsure you are doing this then you will need to calculate the volume of your pond (use our pond calculator to do this) and then the Litres Per Hour (LPH) of your pond pump.

We recommend a box filter for Koi rather than a pressurised filter, with the exception of the Hozelock Bioforce Revolution pressurised filter, as this can keep up with the waste koi produce. Other pressurised filters can struggle to cope with Koi waste, while working with goldfish ponds beautifully.

Keeping filter foams clean will ensure your unit is working at its optimum level, but do not be tempted to clean the bio media at the bottom of your filter as this holds the good bacteria that will help with your pond balance. Many filters have sludge drains, which are fine to use, but scrubbing the bio media clean can have a similar effect to a water change.

UVC units, either stand alone or incorporated in your filter system, play a large part in clearing green water from your pond. Ultra Violet Clarifiers cause the single cell algae to “flocculate” (stick together), before being pushed through your filter foams. This is why it is key to situate any independent UVC units before a filter, because if it is positioned after the algae will disperse and spread out into the pond again. A UVC will need its bulb changed after 6 months of continious use.

If you recently changed a bulb, but are still experiencing green water, then you need to check that the UV is working. The glass sleeve that sits over the top of the bulb can let water in if it is has not been put back correctly with the rubber sealing Oring in place. The only way to tell if a UV light is working is by checking at dusk/ evening to see if there is a blue glow coming from the unit. Most UVC’s have a deadmans switch fitted so they cannot be tested when taken apart (to avoid damaging eye sight), so the unit must be fully put together for you to be able to see the blue glow.     


Green algae will thrive in sunlight, so if your pond is in full sunlight for most of the day then it is probable that you will suffer from green water. In particularly hot weather it may even be worth considering a temporary measure to shade your pond such as a gazebo. Longer term solutions can be to add surface water plants, large bushes/ tall plants to your pond perimeter (try to avoid plants that will drop leaves into the pond) or garden structures. See our blog post on Pond Plants for more information the types of plants that can be installed, both live and artificial plants. 

How to tackle the Green Water

There are several ways in which you can tackle green water. Please see the below suggestions.

  1. Check the current status of your UV bulb. UV bulbs should be replaced after 6 months of continuous use. You should be able to check the bulb to see if it is glowing blue. If it is not glowing blue, then the bulb will need to be replaced. You can see our list of bulbs here.
  2. Plants are really useful when tackling green water. They help to filter water and feed on excess nutrients. Plants, like water lilies, provide shade to the pond which controls the ponds temperature and reduces algae development. A pond without plants will certainly struggle with green water at some point. We would recommend adding a selection of oxygenators, deep water and marginal plants.
  3. You may need to add extra UV filtration to your pond. As stated above, the UV can help bind the algae together so that your mechanical filtration (foams) can filter this out. You will need to match the UV clarifier with the flow rate of your pump. You can see the list of these here.
  4. Apply treatments. Despite tackling the above, ponds will need a little help to fight the green water and there are several treatments to try that you will find on our treatment page.

As always, if you require any specific help with any of the above please contact us.

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