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Pond Help & Information

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Green Water Help (Your pond looks like Pea Soup!)

When a ponds ecosystem becomes unbalanced a condition known as ‘Green Water is likely to occur, caused by the growth of a particular type of algae evenly distributed throughout the pond water. ( Not to be confused with ‘ String Algae’ or ‘Blanket Weed’ which tends to grow in thick strands or clumps).

Also see .....If you have Blanket weed / String Algae 

WHAT'S WRONG ? When a ponds ecosystem becomes unbalanced a condition known as ‘Green Water is likely to occur, caused by the growth of a particular type of algae evenly distributed throughout the pond water. ( Not to be confused with ‘ String Algae’ or ‘Blanket Weed’ which tends to grow in thick strands or clumps ).

Green water algae are microscopic single celled organisms that thrive in warm water with high levels of nutrients, and an abundance of sunlight. For that reason algal problems are prevalent during the summer months.

Algae thrive in new ponds where there are no flourishing plants to provide shade and compete for the available nutrients, and no natural algal predators to keep the growth of algae under control. The result is a pond full of water with the appearance of pea soup.


Algae control is usually the largest maintenance problem in a pond. When setting up a new pond it is normal to have excess algae growth until an ecological balance has been established. This balance is achieved through good water circulation and the right proportion of floating, marginal, and submerged plants, fish, and scavengers such as water-snails and tadpoles. Most plants provide oxygen to the water and all plants absorb nutrients from the water. Submerged plants (also called oxygenating plants) are beneficial because they produce especially large quantities of oxygen and provide supplemental food for fish as well as a place to spawn. Floating plants are most beneficial for providing shade (important to keep the water cooler as cool water holds more oxygen and is healthier for your fish), reducing water evaporation, and providing hiding places for the fish.

Koi ponds are a special case requiring heavy filtration, UVC clarification, and very high levels of water circulation to maintain the pond in good condition and combat the growth of algae. Plants and natural scavengers do not survive the voracious appetite of Koi, consequently Koi ponds must be designed to thrive without the benefit of these natural elements.

When setting up a new pond it is beneficial to add biological agents such as Blagdon/Interpet Biostart

These materials accelerate the growth of the beneficial bacteria known to digest the organic debris that would otherwise accumulate on the bottom of the pond as sludge.

Fish convert oxygen to carbon dioxide and consume insect pests and a little algae.  Scavengers consume algae, decaying plant material and fish waste, thereby providing nutrients for the plants. Fish excrement, once broken down, provides nutrients for the plants. It is a complete cycle with each element depending upon the others to provide what they need to survive.

A little algae is natural, will always occur, and is to a certain extent necessary for a healthy pond. Excessive algae however can be very detrimental to the ecology of the pond not to mention very unsightly. The quantity of algae in your pond depends on the weather, the amount of sunlight, the temperature of the water, the concentration of oxygen in the water and the amount of nutrient and different kinds of plants present. It can be at its worst in the spring when the plants are struggling to recover from the winter but water temperatures and sunlight are on the increase.  The situation may clear up once all the plants have started growing again.

If the pond doesn't clear up on its own, something is out of balance. Algae and water plants compete for the sunlight and nutrients in your pond. Adding some extra plants will in time reduce the amount of nutrients available to the algae and an increase in the amount of surface area covered by water lilies and floating plants will shade a greater area of pond and decrease the amount of sunlight available to the algae. This is of particular importance if your pond is small and shallow. Having too few plants in a sunlit expanse of open water will result in rapid algal growth due to both the direct action of the sunlight and its indirect action in heating the water.

As in many complex systems the secret is balance! So care must be taken to ensure that excessive vegetation does not cover too much of the surface of the pond thereby reducing oxygen absorption and causing oxygen depletion of the pond water. Oxygen is needed in the water to support aquatic life like fish, tadpoles, and aquatic snails, all of which eat algae and help to keep the ponds ecosystem in balance.

If you have a good growth of plants and still have too much algae then perhaps there is an excess of nutrients in the water. This could be caused by a single factor or a number of different factors in combination.

One such factor could be run-off, check to make sure that garden or lawn fertilizers are not washing into the pond when it rains. Perhaps you are over-fertilizing your water lilies or doing it incorrectly. Maybe you have too many fish, and their excrement is causing a nutrient increase. Maybe you are overfeeding your fish. These factors must be considered and corrected if necessary before attempting remedial action to combat the algae itself.


Having corrected the obvious external sources of contamination affecting your pond it is time to apply some more direct corrective measures.

It is advisable to start by testing your pond water so that any obvious deficiencies can be corrected and the effectiveness of subsequent treatments can be gauged. Look in our section on Water Treatments & Pond Chemicals for test kits.

Having tested your pond water, apply the simple corrective measures such as adjusting acidity (pH) and adding some fresh water to the pond, (no more than 10% at a time) to dilute the nutrient levels. If using tap water be sure to ‘age’ it for several days to remove chlorine before adding it to the pond, alternatively treat it with a tap water conditioner such as Blagond Interpet ‘Fresh Start’ to remove the chlorine and neutralize other toxic materials.

The second step would be to treat the pond with an appropriate compound from our range of pond safe products in our Water Treatments & Pond Chemicals section. Blagdon Interpet Green Away is an ideal treatment specially formulated to be compatible with all aquatic life including plants. These will cause the algae and spores to clump together so that they can be easily removed by filtration, or in the absence of filtration, fall to the bottom of the pond. The use of these treatments to control algal growth is no more than a short-term solution as algae can develop resistance and the compounds do not permanently correct the basic imbalance that leads to uncontrolled growth of algae.

The third step would be to install an appropriate ‘UVC’ clarifier to control growth of the algae responsible for ‘green pond’. A UVC unit is basically a tubular chamber inside of which is mounted an ultraviolet lamp. Pond water is pumped through the unit and exposed to ultraviolet radiation causing the algae to stick together in ‘clumps’. The clumps of algae are readily removable by filtration or will fall to the bottom of the pond and form sludge. Excessive sludge may be removed from your pool with a pond vacuum cleaner (see our Vacuum cleaner section) and /or treated with a ‘sludge buster’ such as Blagdon Interpet ‘ Sludge Buster’

 See also the OASE ‘Bitron’ and Blagdon Pro UVC modular range available as stand alone units for out-of-pond applications.

The final and crucial fourth point of attack is biological filtration. Without adequate filtration it is very difficult to maintain the stable balance of algal control, water movement and oxygenation, and nutrient levels that guarantee clear water.

For the satisfactory control of green water algae the combination of UVC clarifier and biological filter is essential. See our filter index. These are biological filtration systems incorporating UVC units. Reference to the appropriate web page will enable you to choose the appropriate system for your particular need.

How UVC's work

We strongly recommend that before proceeding any further you TEST THE POND WATER WITH A POND WATER TEST KIT

(*not a pool kit)  Unless the pond water is balanced you could be wasting your money. the best kit is one that conducts at least 5 tests, pH an Kh being the two most critical.

For suitable pond test kits see our selection  Click here for test kits