In the beginning there was green...

Algae is the most common problem encountered by fish keepers and is possibly one of the more difficult problems to secure a permanent cure for. Algae, being a plant, thrives in a well-lit environment with plenty of nutrients. It can occur at any time of the year but it tends to be more of a problem during the summer, the increase in temperature at this time of year compounds the problem.[This work is copyright © 2000-2021]

As algae can multiply very quickly it can, almost without exception, outperform and outgrow the normal pond plants, and in many cases algae will kill off any submerged plants due to lack of light. Some of the other problems caused by a large quantities of algae can be lack of oxygen at night, especially in warmer weather when oxygen levels are normally low anyway, and fish can get tangled in blanket weed and die (mainly a sturgeon problem due to them not having a reverse gear).[This work is copyright © 2000-2021]

There are two main algae growths, green water (single cell) and stringy (blanket weed / silkweed), normally if you have green water then blanket weed does not grow well as there is no sunlight but if you have clear water then the blanket weed will grow and keep the green water away.[This work is copyright © 2000-2021]

The most common mistake made by pond keepers is to empty the green pond and refill with fresh water this unfortunately starts the process all over again as the introduction of fresh water means new nutrients are made available and the cycle starts again.[This work is copyright © 2000-2021]

Algae Cures

There are many options for the removal of algae from ponds and, like old wives tales, many are just pretty packets with empty promises. Many chemical treatments for the control of 'green water' or 'blanket weed' are deadly to sturgeons so beware! Natural treatments will usually take a little longer to work but they are much safer for your fish.[This work is copyright © 2000-2021]

The quickest and easiest way is to add chemicals. There are many different approaches chemical treatments use to destroy the algae; some dye the algae to stop it absorbing light, stopping photosynthesis so that the plant starves; some are aquatic herbicides, weed killer in gardening terms, which at low levels effectively kill algae but if overdosed can and will affect the other pond plants, water lilies being the most susceptible to these treatments. [This work is copyright © 2000-2021]

More natural methods, such as balancing the number of fish against the plants, can work but are usually very slow to become established and the number of fish a pond will support is normally very low.[This work is copyright © 2000-2021]

When using chemicals it is essential that you follow the instructions very carefully. As the algae starts to die it will pollute and deoxygenate the water this can lead to fish deaths, follow a few simple tips to avoid any problems:[This work is copyright © 2000-2021]

  • The most important factor is to remove as much of the algae as possible before you treat the pond. This will increase the effect the treatment has and reduce the possible side effects (see below)
  • Make sure the pond pump is running at its highest setting, this will create extra oxygenation and pump the water through the filter quicker, thus filtering out the worse of the dead algae.
  • Clean the pump regularly during treatment, as it will clog with the free-floating dead algae.
  • Put a fine filter mat in the filter this will collect the dead algae quicker and it can be washed every day with out up setting the biological section of the filter, or clogging it up.
  • Reduce the amount of food the fish are getting this will reduce the amount of oxygen they require and lower the load on the filter, as they will produce less waste.[This work is copyright © 2000-2021]
  • Once the pond has cleared vacuum the dead algae from the bottom of the pond, this will stop the nutrients from re-entering the cycle.

Natural treatments

Plant extracts are useful for the natural control of algae, the most common being barley straw. Barley straw is available as a liquid extract or as natural barley straw pads. [This work is copyright © 2000-2021]

Barley straw pads need to be placed in the pond so that they can rot down and the enzyme that affects the algae can be released. The whole process can take up to 6 weeks to start working and the pads need to be added well before the main algae season starts in the spring. As the straw rots away new pads need to be added before the old one runs out so to keep the cycle going.[This work is copyright © 2000-2021]

Barley straw extract is a ready to use liquid that has been produced with the enzyme in it. It needs to be added every month and it is not generally as effective as the straw its self. The only draw back with any of the additives, chemical or natural, is that they do not work in every pond and may not even work from one year to the next as the algae slowly develops immunity.[This work is copyright © 2000-2021]

Anti Blanket Weed treatment from Orchard Fisheries
Anti Blanket Weed treatment

The Anti Blanket Weed treatment from Orchard Fisheries is a simple water additive that works by altering the water chemistry to reduce blanket weed growth in planted ponds. The treatment does not contain algicides and is safe to use in planted ponds and with all fish species (inculding sturgeons). Anti Blanket Weed will not knock back filters and is not temperature dependent so can be used at any time of the year.[This work is copyright © 2000-2021]

Buy Anti Blanket Weed treatment from Orchard Fisheries »[This work is copyright © 2000-2021]

As with any natural products it doesn't always work for every one, it is a matter of trial and error for every pond. They don't affect the environment as chemicals do, so waste water can be used on the garden with out fear of killing the plants.[This work is copyright © 2000-2021]

Electronic Algae Control

The most reliable method of controlling green water (not blanket weed) is the use of an Ultraviolet clarifier (UVC). This method has proven its worth for many years and is the best long-term cure for the condition. They are always installed with a biological filter and pump and need to run 24 hours a day, 7 days a week to get the best results. [This work is copyright © 2000-2021]

Read more about Ultraviolet clarifiers »[This work is copyright © 2000-2021]

The control of blanket weed with electronic devices as proven to be a very difficult area. All the products I have tried none have proven very reliable and those that do work only work for a very short period of time, as the algae adapts to its new surrounding very quickly and soon nullifies the effect of the electronics. Most of the electronic devices sold are in fact re-labelled anti scale units from the plumbing trade. So if you want to try one get them at the plumbers they are cheaper![This work is copyright © 2000-2021]

Non chemical treatments

Another well-sold anti-blanket weed device is the permanent magnet that is placed in the pumped water line. These have a similar effect to the electronic devices and are about as effective, to prolong their effectiveness they can be removed when they stop working and replaced in 3 or 4 months time and they should work again for a time. The only way to see if these devices works for you is to try them some do most don't. Ever the sceptic, I have noticed the old lifetime warranties have soon gone now the manufactures realise they don't work for long let alone a lifetime![This work is copyright © 2000-2021]

Vegetable filters

The best and most reliable method for getting rid of green water and blanket weed is to use plants, it costs nothing to run and improves the water quality at the same time, something the other products can not offer.[This work is copyright © 2000-2021]

vegetable filter
Our veggie filter

The basic principle for this is the use of plants to consume the excess nutrients before the algae can get to it. A simple plant filter is called a 'vegetable filter' it is a very basic form of hydroponics and uses water plants to remove the nutrients from the water in order to grow. The best plants are the fastest plants that will grow in the prevailing conditions. One I find to be very good is Watercress (Nasturtium officinale). Not only does it grow very quickly, it also uses up a very large amount of nitrogen in the process and it can be fed to the fish, providing a free meal for the fish as well. A small bag from the salad section in your local supermarket is a cheap and easy way to get started, it will quickly root and grow in the water.[This work is copyright © 2000-2021]

The vegetable filters design is very simple and requires little maintenance and works very well. The best position for the filter is in full sun, as the plants will do better with more light. It is best placed after the bio filter, as this will be the highest concentration of nitrate, plant food, because the bio filter converts the fish waste to nitrate. The filter needs to be shallow with a gravel substrate that the plants can root into but no soil, as this will stop the plants from doing their job. The slower the water flows through the filter the better, as the plants will have more time to extract the nutrients.[This work is copyright © 2000-2021]

As this section of the filter has a lot of plants in it protection from koi will be need, as they would destroy it if they could get in. If you have no room for a separate vegetable filter then it is possible to grow water plants in the waterfall or even in the top of the bio filter.[This work is copyright © 2000-2021]

The vegetable filter method can take time to start working but do not be tempted to use chemicals to help as they will certainly effect the plants you want to grow as well. If the need for an 'instant' cure is important then the addition of floating plants will help to speed things up as they not only use the nutrients to grow but also block the light from the algae, and they can be thinned and removed as necessary, but only during the summer months as most floating plants are not hardy.[This work is copyright © 2000-2021]

Read more about vegetable filters »[This work is copyright © 2000-2021]

Pre-water treatment

algae controller
Algae Controller

New items have appeared on the market since writing this article. Once such device is the Algae Controller, a simple unit which pre-treats the tap water before it enters the pond, is a copper zinc alloy which effects the way the calcium crystals are 'shaped' this stops the blanket weed using it as a mineral source. It is very effective but requires the pond to have numerous water changes to get the effect, but once done it can work for up to 6 months (normally 3 months) and when the algae starts to grow a quick partial water change is all that is needed to stop it again. As far as I have seen in my own systems the algae does not become immune to it. It does have one side effect, it kills Watercress, but no other plants I have tried with it have been affected.[This work is copyright © 2000-2021]


What ever you decide to use to control the algae there is one thing that needs to stay in your mind the main cause of the algae is the fish (well the waste they produce), the more fish you have the more food for the algae. It must be remembered that the reason the algae is growing is because nature is trying to reduce the toxic waste levels in the pond before they can harm the fish. The growth of algae should be considered a good thing, as it is saving your fish, all that is needed is to replace the algae with another plant that is more manageable and easier to control.[This work is copyright © 2000-2021]

Written by Terry White & Graham Quick[This work is copyright © 2000-2021]