Green Water Control

22 Aug 16:58

Green Water Control

Green water is made up of millions of tiny, free floating single celled algae. These are photosynthetic and contain chlorophyll, which gives them their colour.

What problems can it cause?

Green water obviously looks unsightly and stops you from enjoying your fish. It also can have some negative impacts on the health pond as well.


Algae can deplete oxygen levels in the pond. Like all green plants, algae converts sunlight into food by photosynthesis. During the day the algae removes carbon dioxide from the water and produces oxygen. However, at night this process is reversed and oxygen is taken out of the water by the algae. This can be the reason for sudden livestock deaths that occur in the mornings.


Algae will naturally cause slight pH fluctuations during the day. The reason for this is that the oxygen produced by the algae during the day causes the water pH to increase. At night as photosynthesis reverses, the carbon dioxide being produced causes the water pH to decrease.  This pH fluctuation does not usually cause many problems. However, in ponds with a particularly high concentration of green water, this pH swing between lower night time pH and higher daytime pH can be significant and may cause stress and damage to fish.

What makes algae grow?

In order to grow, algae requires warmth, sunlight and nutrients (particularly nitrates and phosphates). You often find that algae dies back in the winter when the water is cooler and the sunlight is weaker; but creeps back in the spring as the pond heats up and the fish start feeding and producing waste that the algae uses as food. Another source of food for algae is sludge in your pond filter and at the bottom of your pond.


Good housekeeping and pond design

Maintaining your pond regularly and correctly will go a long way to reducing algae problems. Ensure that you remove sludge collecting in any filter boxes on a weekly basis and also regularly removing detritus such as dead leaves on the bottom of your pond. You can use a vacuum for this, or simply use a fine net to fish it out on a regular basis. The idea is not to have a spotlessly clean pond, but to reduce the build up of organic waste a bit.

When building your pond, consider the aspect, try not to have it in full sun all day long as that is just asking for an algae problem. If you have no natural shade in your garden, you could consider introducing some in the form of a sail or a pergola. Garden/pond features like these can look great as well as help to reduce direct sunlight.

Natural Cures

Plants are a great way of reducing nuisance algae as they will feed off the nitrates that encourage algae growth.


Barley straw is commonly used as it produces an enzyme as it breaks down which helps to control algae. We find that this works very well in some ponds and not so well in others. It will generally work better in ponds with a low stock of fish.

Chemical Treatments

There are 2 main types of treatment: quick fixes and slow burns.

Most of the liquids on the market are what we tend to call quick fixes. These are quick and effective, temporary solutions. Most are flocculants - so they lump the algae together so it becomes large enough for the filter to catch it. If you have an unfiltered pond, it will lump together and fall to the bottom of the pond. In this situation you really should remove it with a net.  Do remember to read the label of chemical treatments to ensure they are safe for your fish and wildlife.

There are a few other products that last a bit longer and are designed to go in your filter as a slow release algaecide. These work pretty well, but do not produce results as quickly as the flocculants.

Filtration Methods

By far the best, most reliable way to get rid of green water is to use a UV clarifier. The UV light will club together the algae cells into larger lumps, which can be in turn removed by filter sponges. UV lights usually last for one season. We normally suggest that you replace your UV bulb in early spring. You can reasonably expect them to remain active until the autumn.

What size UV?

We recommend working on 1 watt of UV for every 100 gallons (450 Litres) of pond water. However, if you have a very heavily stocked pond, or it is in full sunlight, you may wish to go bigger.

Alongside volume, another important factor to consider is the flow rate through your UV. We always recommend you use a pump that turns over the volume of your pond every hour. The bare minimum should be once every two hours. This is because algae can reproduce so quickly.

What type of UV Filter?

There are many different types and selecting the right type for your pond will depend on factors such as: the size of your pond, the aspect of your pond and it’s stocking density. Other things that also influence your decision may be the physical size of the filter and the frequency of cleaning.

The most common types of filtration used in garden ponds are: all in one filters, gravity filters and pressurised filters.

Author & Photo credit - Parkers Manager Patrick Davies.