Ponds, like us are living systems and much like our children, they don’t always behave the way we want them to.

On this point, I am reminded of the pond and waterfall that I built for one of my clients. They decorated it with some aquatic plants and a handful or so of goldfish. Although the water went through an initial ‘green phase’, the pond soon cleared up. They told me how they spent many enjoyable hours with the relaxing sound of the waterfall and lazily admiring the fish. Everything about it was wonderful that first summer. Next spring they were rewarded with a pair of Mallards that decided to nest nearby and spend time playing in their pond. This completed their picture perfect pond! It was mid-June when they first noticed some green clouds in the pond and a few weeks later there was green goo hanging down their waterfalls. That’s when they decided to call me...

I told them that it was string algae (also known as filamentous algae) and not to worry because It will not harm the fish, as long as it does not take over the whole pond. It is mostly an unsightly problem, especially in the waterfall. One simple thing that you can do to immediately improve the waterfall is to turn off the pump for a couple of hours during a sunny afternoon. String algae hate this because it needs water to survive. It will come back eventually, but you will have a reprieve for several weeks and when it does, you can dry it out again. A word of caution here: do not keep the pump off much longer than a couple of hours because the lack of oxygen will harm the beneficial organisms that keep your pond healthy and this will only magnify your problems.

Filamentous algae grow by forming long threads that can intertwine with each other to form dense mats. Being rather lazy, this algae prefers shallow moving water where sunlight is most intense and nutrients are delivered to it in abundance. So if you have a creek or a waterfall that’s a perfect situation for it. There are hundreds of different species and since most can reproduce by microscopic spores, it is only a matter of time before they show up in your pond. There are also some slimy types that can form floating mats on top of ponds. Luckily, the types that are most common in the South Okanagan are more like fine hair than slimy and they are anchored to a substrate. Because it is anchored to the liner or rocks, it doesn’t pass through the UV chamber to get wiped out by the ultra violet rays.

In order to control it, we need to understand what algae thrives on. Algae need nutrients (mostly nitrates and phosphates) and sunlight. Knowing this, there are several things that you can do to discourage its growth, and to make this easier to understand, we will group these into three categories: mechanical, biological and chemical. Often it is best to use a combination of these.



Mechanical control simply means you physically remove it, usually by twirling it either around your hand or you can use something like a plastic garden rake. Some people find that a toilet brush works really well for this. You will get instantaneous results, but it will most likely come back in a few days and it is difficult to get all of it. The good thing about this method is that you are also instantly removing nutrients from your pond and I like the feeling of satisfaction that it gives me.



Alex (one of my customers) recently sent me this picture of string algae he had just removed ‘mechanically’. It reminds me of a proud angler holding up a trophy fish.

Koi and Goldfish like to eat string algae, especially if you don’t feed them too much. It is a healthy addition to their diet and they will spend hours picking away at it in ponds that

are not over run by it. There is also a species of fish that is supposed to have a ravenous appetite for string algae and that’s the Blue Tilapia. Unfortunately it is not winter hardy, so you would have to bring them inside.




The biological control is the most complex and combines several strategies but it also the most efficient because once your pond is balanced, it will take care of itself, as if it were on cruise control. Basically the goal is to reduce the amount of nutrients and sunlight that gets into your pond.

If your pond is located in full sun, you can to create shade with some sort of structure like an arbor, or use cover plants on the surface of the water. Water lilies are one of the best plants for this, but you can also use floaters such as Water hyacinths, Water Lettuce or Duck Weed. A good rule of thumb is to cover 50 to 60% of the pond surface. If you go over this, it takes away from the aesthetics of the pond and it also starts to interfere with oxygen diffusing into the pond. By the way, trees are not ideal because they drop so many leaves and other nasty things that foul the water.

Another way to reduce the light is to add a dye to your water (usually blue, purple or black) which you can find in water garden stores. These are safe for fish and plants except the oxygenators that live under water and need the light to survive. The only other drawback with using these dyes is that you will be looking at your fish through coloured water.

There are several ways to starve the algae of nutrients. You can start by limiting your fish population to a reasonable level and cutting down on the amount of food that you give them. This means that you need to resist the urge to feed your fish every time they come to the surface begging for food. Also try to limit the amount of plant organics that end up in your pond by deadheading your aquatic plants. Skimmers pick up whatever falls on the surface of the pond (leaves, grass clippings, etc.) before it has a chance to sink to the bottom and start decaying.

Another source of nutrients is the sludge that accumulates at the bottom of your pond. Some people like to vacuum their pond on a regular basis, but with my limited time, I prefer to empty the pond once a year and remove any accumulated sludge at this time (see article on our web site for specific instructions on this).

Be careful when feeding your aquatic plants that the fertilizer does not end up in the water and finally, Don’t allow runoff water to enter into the pond as it can be can be a major source of nutrients.

Taking these preventative steps will help. Now, the next step is to remove nutrients that are already in the water. Plants use up nitrates and phosphates. So as a rule, the more plants you have, the fewer algae will grow because the plants compete with the algae for nutrients. Some plants that are fast growing like cattails, water hyacinths and certain irises are more efficient at this than the less aggressive plants. Oxygenators are another group of plants that use copious amounts of phosphates and nitrates, but you must have enough of them to make the difference.

Aside from plants, some of the beneficial microbes (like Microbe-Lift that I recommend to clean and help balance the pond) also contain bacteria that are photosynthetic and use up nutrients. With these especially, it is important to have good oxygenation so that these micro-organisms can multiply freely and thrive.

There are also products with names like ‘Phosphate Remover’, or ’Phosphate X’ that remove phosphates. My experience with these is that you have to use a substantial amount over a period of time to get results.

Every pond is a mini ecosystem and each one is different and unique depending on the combination of a multitude of factors that affect it. Examples of these are the amount of sunlight, the chemistry of the water, the types of plants in and surrounding your pond, the microorganisms that populate it, temperature, depth of the water and even the type of food that you feed your fish. If you follow the guidelines outlined above and give it a little time, it is likely that your pond will naturally reach a point of equilibrium that does not include string algae.

Sometimes however, you can do all the right things and still be plagued with algae problems. Sometimes the well water itself stimulates the growth of string algae with its high calcium content. Many of the water sources in the South Okanagan are alkaline which tends to aggravate the algae problems. So, for all those situations where enhancing the natural biological elements is not enough to clear the pond, there are alternatives that I classify under the heading of ‘Chemical’ control.



These are products that actually kill the string algae; however it is important to mechanically remove as much algae as you can before you use these products. If you don’t, the dying algae will quickly decompose and load the pond with a new wave of nutrients, but what is even worse is that the breakdown process consumes the pond’s oxygen supply. If there is too much dead algae the fish can succumb from oxygen depletion.

The least invasive treatment is the use of barley straw. In the spring you simply place a bale of straw in a part of the pond that has flowing water. There are microorganisms that colonize the straw after a few weeks and they produce a substance that destroys the algae. I have clients that swear by this and get great results, but it does not seem to work in all ponds. You have to try it to see if it works for you. I like the idea that it is natural and very safe. If you don’t like the idea of straw in your pond, they now make a concentrated liquid extract.

Salting your pond to .3% (3 Lbs. non-iodized salt per 100 gallons applied over a period of 3 days) will get rid of string algae, but beware that many of the aquatic plants cannot tolerate this level of salt.

There is a product that gets instantaneous results by literally dissolving the algae. It is called D-SOLV and it works well for many pond owners, but you have to treat your pond on a regular basis in order to stay on top of it. I like to use it to treat the pools of water that remain in the waterfall if I am drying out the waterfall.

My favorite ‘chemical’ solution to keep string algae in check is a product called Pond Balance. You have to do three treatments 10 days apart, but by the end of the third treatment it’s a done deal. Everyone that has tried it is impressed with the results. It gets rid of their string algae and it is safe for fish, plants and birds.

There is also a device that beats string algae. It is called an ionizer and it works by passing a weak electrical current between two copper electrodes. This process liberates copper ions into the water which kill the string algae. You have to monitor the concentration of copper in the pond and adjust the voltage accordingly to maintain safe levels of ions that will not harm the fish. I particularly like this system for pondless water features that don’t have fish. There is a lot more to say about ionizers, but I will save that for another article.

My clients who had called about the string algae decided to reduce their fish feeding to once a day. They added a couple of water lilies, a dozen or so water hyacinths and a couple of cattails in large planting baskets. They let their waterfall dry out for a couple of hours. This was followed by weekly sessions of hand removal of the string algae and regular use of Microbe-Lift. Within a month the balance was restored and they were proud and happy to have gotten through the teenage phase of their pond.

I hope that you now have a better understanding of how this green string-like stuff grows and know different strategies to keep it under control. Remember that every pond is unique and it may take some experimenting before you find the right combination that works for your pond. With some perseverance on your part you will learn to tame your pond’s behavior and it will reward you with much enjoyment.


Written by Michel D'estimauville — February 10, 2013

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