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Pond Help & Information

Need a hand working out the right gear for your project? Browse our range of help pages or submit a help request directly to our specialists.

Blanket Weed or String Algae Help (long stringy or cotton wool like algae)

Blanket weed, thread algae, string algae and filament algae, are common names for a variety of different species of filamentous algae.

Also see .....If you have Green Water/Pea Soup


Blanket weed is very often seen clogging pond filters and entangled with pond plants. It grows in dense green masses and quickly becomes an unsightly nuisance if steps are not taken to control its growth.


When setting up a new pond it is normal to have excess algae growing pending the establishment of an ecological balance. Established pond can also become over grown with algae due to a build up of nutrients or other factors upsetting the established ecology. An ecological balance is achieved through the growth of a balanced proportion of floating, marginal and submerged plants, fish, and scavengers (snails and tadpoles). Most plants provide oxygen and all plants consume nutrients from the water. Submerged (also called oxygenating) plants are beneficial because they produce quantities of oxygen and provide supplemental food for fish as well as a place to spawn. Floating plants are most beneficial for providing shade to keep the water cool as cooler water holds more oxygen and is healthier for your fish, reduce water evaporation, and provide hiding places for fish.

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

Algae are a normal part of the ecology of ponds, will always occur, and are to some extent necessary for a healthy pond. An excessive growth of algae however can be very detrimental and very unsightly. The quantity of algae in your pond will depend on the weather, the amount of sunlight penetrating the water, the temperature of the water, the concentration of oxygen in the water, the amount of nutrients present and the number and variety of plants present. Algal growth is likely to be at its worst in the spring when water temperatures and sunlight are on the increase and the plants are still struggling to recover from winter. In time the plants may flourish again and the situation may improve.

If the pond ecosystem doesn’t reach a satisfactory balance of its own volition, something is out of balance. As the algae and water plants in your pond compete for the same sunlight and nutrients, adding some extra plants may be beneficial. This will reduce the nutrients available to the algae and the increase in surface shade afforded by additional water-lilies and floating plants will decrease the sunlight available to the algae. This is of particular importance if your pond is small and shallow. Having only a few plants in such an open expanse of water exposed to direct sunlight, will result in warmer water and increased algae production.

Little or no plant life can be sustained in Koi ponds as these fish are voracious eaters of plant life and prefer to live in open water. It is not possible to attain a ‘natural’ ecological balance in a pond where Koi are in residence, and heavy filtration and high water circulation rates are required to keep the water clear and in good condition. Blanket weed is very much at home in clear water with lots of available sunlight and can be a major problem to Koi keepers.

On the other hand 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. Lack of oxygen in the water encourages algal growth as algae, like plants, need carbon dioxide to survive. At higher oxygen levels, carbon dioxide levels will be lower and the probability of excessive algal growth will be reduced.

Oxygen is also 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 are excessive nutrients in the water. This could be caused by a single factor or a by a combination of several different factors. A common problem is run-off from adjacent lawns and gardens. Check to make sure that fertilizers are not washing into the pond when it rains or when irrigation systems are operating. Perhaps you are over-fertilizing your water-lilies and other water plants or maybe you are doing it incorrectly. You may have too many fish so that their droppings are causing an increase in nutrient levels. Maybe you are overfeeding your fish ?.

These factors must be carefully 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. Start by physically removing as much Blanket weed as possible by winding it onto a suitable stick ( a split bamboo works well . Leave the algae at the side of the pond for a while so that the creatures living in it can return to the pond.

It is advisable to test 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 from various manufacturers.

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.

The second step would be to dose the pond with an appropriate treatment from our range of pond safe products. Both Blagdon Interpet Pond Balance and Blagdon Interpet Blanket Weed Algaway are suitable treatments specially formulated to be compatible with all aquatic life including plants. These treatments will cause the algae to die and fall to the bottom of the pond where they will form a sludge. The use of an additive treatment to control algae is no more than a short-term solution as algae will develop resistance to these materials relatively quickly.

Excessive sludge may be removed by treating the pond with a ‘sludge buster’ such as Blagdon Interpet Sludge Buster, refer to our section on ‘ Water treatments & Pond chemicals’.

An alternative to treatment with a biological sludge buster is mechanical sludge removal with a pond vacuum cleaner (we offer both Electric and manual models)

Dosing your pond with additives does nothing to correct the basic imbalance responsible for the uncontrolled growth of algae. Longer term strategies are required to ensure to that outbreaks of excessive algal growth do not reoccur. It is good practice to test your pond water regularly to ensure that pH and nutrient ( Nitrate ) levels are at optimum levels. Adequate filtration is essential to maintain the stable balance of good oxygenation and controlled nutrient levels essential for a healthy pond. Excellent biological filtration is provided by the range of pressure and gravity fed filters listed in our filters section. See OASE Filtoclear, OASE Biotec, and in our pressure filter chart or Gravity filter chart

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 and Kh being the two most critical.

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