Keeping the water fresh and reasonably free of pond algae is important for garden pond upkeep. Algae turn the water green and muddy, and it can grow on rocks and around the margins, where it can be unsightly. Excessive algae development is not just ugly but also harmful to the pond’s other aquatic life.

Before you panic, if you notice an algal accumulation in your pond, remember that algae are a fully typical element of any organic pond ecology. It’s fine to have a tiny amount of algae in your water feature! It is, in fact, an indication of a healthy environment. Algae become a concern only when their development becomes enormous and difficult to regulate.

When temperatures rise and daylight hours lengthen every spring, an algae bloom develops in practically every koi pond. Algal blooms, such as green water that obscures koi or unattractive string algae that appears out of nowhere, appear to be an unavoidable feature of pond life. Therefore, it will be critical to control pond algae successfully.

What exactly are algae?

The term “algae” refers to a wide variety of aquatic plant-based creatures, ranging from tiny algae found in ponds to huge seaweeds like the 100ft big kelp.

Several types of algae are present in garden ponds, including “blue-green” algae, cyanobacteria, and normal green, brown, and red algae.

Like many other plants, pond algae grow via photosynthesis, which involves converting sunlight, carbon dioxide (CO2), and nutrients into biomass, which is subsequently utilized for growth.

Where does pond algae originate?

Algae, including cleaned tap water, may be found in practically any type of water. Water companies will kill most algae during sterilizing, but a tiny amount will survive.

Algae reproduce by producing spores, and these spores have extremely resistant shells that allow them to survive even rigorous water treatment. So, for example, when you add water to your garden pond, a single algae or spore might start quick algae bloom if the conditions are favorable.

This is why you frequently hear about big algal issues with new pond constructions, particularly ones that have not been cycled properly. This is because a new pond’s algae have little competition for nutrients and exceptionally clean water for quick photosynthesis.

Furthermore, because new ponds are not cycled, there are no established beneficial bacteria populations to break down dead algae, resulting in increases in compounds that disrupt the natural cycle and help in algal development.

Why Does Algae Bloom in the Pond?

An algal bloom is an overpopulation of algae. Algae thrive in warm, damp, nutrient-rich environments. Therefore, many necessary conditions for algae to develop may be found in any fish pond that receives some sunlight.

Algae grow in almost every pond at some time. The goal is to maintain such levels to a low by developing a balanced ecology, which takes time. Nature attempts to establish equilibrium by forming algae.

  • Excessive sunlight penetrates the water surface, boosting light and raising the temperature of the water.
  • Excess Nutrients include fish waste and uneaten fish meal. The pond is overcrowded.
  • Inadequate Filtration- the majority of ponds are under-filtered! Inadequate filtration is frequently selected when ponds are first created, mainly owing to the additional expense. Additional filtration (pressure, bog, and bead filters) can be added to your pond as needed and within your budget. Plants and helpful bacterial treatments can also improve your pond’s filtration capabilities.
  • Ponds that are still- Aeration is essential for all pond life. We attempt to achieve acceptable dissolved oxygen levels by circulating water from oxygen-bubbled pumps and aerators. This promotes the growth and well-being of our plants and fish, as well as helpful bacteria that work to dissolve excess nutrients and hazardous compounds such as ammonia.

There are two forms of algae.

1. Phytoplanktonic algae

green grass on brown sand

Phytoplanktonic (free-floating, suspended algae) kinds include those that create “green water” or “pea soup” water conditions.

What exactly is Suspended Algae?

Suspended algae are single-celled algae that proliferate fast, rendering water green and murky if left uncontrolled. Because algae cells are so minute, most filtering medium is too porous to collect and remove them. Because of the murky green water it creates, this type of algae is sometimes referred to as ‘pea soup.’


A water imbalance often triggers suspended algae blooms. Too much sunshine and plenty of nutrients can also contribute to floating algae development. Excess nutrients in the pond are frequently caused by overfeeding, rainwater runoff, overstocking, or a lack of effective pond filtration.

Suspended algae blooms are common in the spring, but the water temperature stays low when the season changes. Because low water temperatures prohibit aquatic vegetation and fish from being active, algae will be the only creature actively contributing to the ecosystem at this time.

As a result, the suspended algae aren’t fighting for food with fish and aquatic plants, allowing them to grow.

This sort of algae may be the most prevalent problem for pond owners. In the spring, ponds frequently resemble a giant vat of pea soup, which is not how koi owners want their ponds to seem.

Aside from being a spring bloom occurrence, these single-celled algae are frequently connected with freshly built ponds since the filter hasn’t had enough time to build a suitably big bacterial population.

2. Benthic algae

2) Benthic (attached) algae such as “string algae” or “horsehair algae,” “water net,” and “blanket weed.” String algae can be difficult to remove. These algae may remain dormant in a dried form for years until it is brought to water, at which point they will grow.

What exactly is String Algae?

String algae are filamentous algae commonly connected to water feature surfaces like rocks and plants. If left addressed, excessive string algae development could be aggravating.

As a result, it is important to take the necessary precautions to prevent excessive growth in your water feature.


String algae blooms, like suspended algae, are created by a water feature imbalance. Excess nutrition and sunshine are common contributors to their growth. String algae spores are highly robust, and as a result, this type of algae may proliferate pretty readily, as you may have seen.

Another issue is that when you manually remove it from your pond (the ideal method), the process of removing it causes it to release reproductive spores into the water, restarting the cycle.

Because string algae produce a lot of dissolved oxygen, it tends to accumulate bubbles caught in its “hair,” Before long, a vast, unattractive mat of the material floats to the top, decreasing the attractiveness of your pond even more.

Of course, anything that increases the amount of dissolved oxygen in your pond is a good thing, right? Yes, until it dies, drops to the bottom, and is broken down by bacteria that consume oxygen, reducing the dissolved oxygen in your pond.

Is it necessary for me to eliminate algae from my pond?

Even if you can’t see it, algae will be present in all ponds. Problems emerge when algae levels get high enough to disrupt the pond’s natural nitrogen cycle, generating surges in ammonia, nitrates, and a significant decrease in dissolved oxygen.

If you don’t have fish in your pond, this isn’t an immediate concern, but increased algae levels can create sludge accumulation, resulting in an unpleasant stench. They may also cause water clarity issues, turning ponds a pea-green hue. Pond plants will also have to fight for nutrients with the algae, so if the algae’s volume is too high, the plants will suffer.

If you have a fish-free backyard pond, the decision to eliminate algae is purely ornamental. However, removing algae is recommended if you want pure water or to enhance plant development. The fewer algae in your water, the cleaner it will be, and the more nutrients your plants will have to grow strong. However, if you have fish in your pond, algae development must be regularly checked since a rise in growth can severely injure pond fish in a few days.

Are algae harmful to pond fish?

green trees beside river under blue sky during daytime

Small levels of algae do not hurt fish; certain fish, such as koi, like eating specific forms of algae! Problems arise when algae growth surges abruptly or slow development is allowed to continue unabated, increasing algal populations.

Like other photosynthesizing plants, algae will consume sunlight and CO2 throughout the day and create oxygen as a byproduct. Pond fish require adequate amounts of dissolved oxygen in the water to breathe, so why are algae hazardous to fish?

Algae will suck nutrients, minerals, and oxygen from the water to sustain themselves during the night when there is no sunshine. At the same time, as algae compete for nutrients with one another (and with pond plants), some will die off as nutrients are absorbed.

When algae die, they begin to degrade, and the bacteria that trigger the breakdown process require a lot of oxygen to function. This process will continue as algae grow, with dissolved oxygen levels quickly dropping as algae grow and dies. Because the oxygen generated by algae during the day cannot constantly keep up with the oxygen lost during decomposition during the night, fish will gradually drown.

If you have fish in your pond and algae has begun to bloom, you must take precautions to ensure that the growth does not go out of control. A pond does not have to be completely green to be dangerous; even a tiny green tinge to the water is enough to cause worry, especially during the warmer months when oxygen levels are typically lower.

We recommend eliminating or minimizing algae in fish ponds when you see a green tinge to the water or when you notice big amounts of string algae development on the pond’s sides.

Can I get rid of algae quickly?

The actions alluded to here aim to slow and eventually stop fibrous algae’s growth by biological means. The entire procedure will take many months. This period can be cut in half by using an algae-fighting solution. Please keep in mind, however, that this type of product just combats the symptom algae and does not address the root cause.

Determine the Source of the Algae Growth

By checking the quality of your water, you may look for probable sources of string algae. If algae growth becomes a concern, it is important to look past the algae and mat and deeper into the pond chemistry. Algae blooms are mostly caused by high pH and phosphorus levels.

Clearing algal blooms, excessive plant growth, overstocking fish, and entering foreign materials (untreated concrete, rocks containing limestone, or calcium/granite) might create high pH. Farm fertilizer leaching is the most prevalent source of elevated phosphorus levels in pond water. Iron is also a significant contributor, as are grass clippings that end up in the pond after mowing the yard. Remove any green grass blades as soon as possible.

Prevention tips

gold fish in water

The following are some methods for preventing algal development in your pond.

Feed your koi less

Using high-quality fish food will also reduce algae development since the meal will be thoroughly digested, allowing fewer nutrients to flow through the fish.

Add algae-eating fish

We’ve written an article that covers the different types of algae-eating fish you can add to your pond that can eat most of the algae in your pond.

Increase Shade coverage

Because sunlight is an important component for algae to develop, you may effectively minimize part of the potential algal fuel entering your pond by shading it somehow. One method is to use trees, which is an old-fashioned method. Try growing trees around your pond that will give shade.

Aside from aerial shadow, there is also underwater shade. For a long time, pond owners have put aquatic plants such as lilies in their ponds to generate shade and make your pond more aesthetically pleasing.

Regarding aquatic plant coverage, the “magic” number to aim for is 60-70 percent surface covering. Non-toxic coloring additives (dyes) that tint your water a certain hue and restrict the available light in your pond are another technique to reduce light penetration.

Many pond owners consider this to be their “go-to” option. They normally endure a long time and come in various dye colors. Unfortunately, reduced sunlight penetration reduces algae’s capacity to flourish! Here are a few nice dye options to consider.

Add Plants

In the wild, fish provide nutrients that plants take, leaving little room for algae. Many garden ponds, however, do not have enough plants to utilize all of the nutrients provided by the fish. This results in an overabundance of accumulation and creates an ideal setting for fast algae development.

Whether you’re just starting and want to avoid algae problems or have a current problem that needs to be addressed, you’ll want to start by increasing the number of oxygenating plants on the pond’s surface. This is possibly the most basic and long-term approach to clean and clear water.

Floating plants, such as lilies and lotus, offer shade and prevent direct sunlight in the pond, limiting algae formation. In addition, submerged plants that release oxygen into the water, such as anacharis, hornwort, and parrot’s feathers, should be included. One bunch of six to seven strands of the oxygenating plant can be placed on every two square feet of water surface and buried by tying to a rock or plant in a soil container as a guideline.

All aquatic vegetation consumes nutrients, starving algae. Green water may appear after the initial plant insertion, but it will only stay for a short period. Established marginal plants can be put along the pond’s perimeter or in shallow parts. These are also beneficial in terms of absorbing nutrients and providing shade.

Any pond plant will fight for nutrients with algae, so you can’t go wrong with most options. Plants like hornwort and water lettuce, on the other hand, are excellent oxygenators, are simple to grow, and are resistant to seasonal changes — two excellent places to start.

Lower Nutrient Loads

Because nutrients like nitrates and phosphates are essential for algal growth, limiting and eliminating these components can significantly impair algae’s capacity to thrive. This is accomplished by not overfeeding your koi, monitoring water chemistry, and modifying as needed.

Make sure your pond isn’t exposed to fertilizer runoff, which can transport a lot of phosphates. Instead, ensure you have lots of filtration and helpful microorganisms to help with vitamin absorption, etc. To lessen nutrient loads, you may need to do repeated water changes.

If this is the case, ensure that the water adjustments you make are moderate so that your pond does not experience a major pH swing, which might injure your koi. This page explains how biological cycles function in greater detail.


Salt appears to be a go-to treatment for various issues in the world of koi keeping, and it turns out that it can also help manage algae blooms. However, one caveat to using salt to treat algae in your pond is that excessive salinities will also hurt or destroy your aquatic vegetation.

For example, popular plants such as water hyacinth and lotus will begin to die back at 0.10 percent, but water lily will not die off until 0.5 percent, and to successfully deal with algae, you should aim for 0.25 to 0.30 percent. Then, based on your resident aquatic plant species, you must decide whether salt makes sense for your algae concerns.

Using the koi-online care calculator is a smart approach to estimating the appropriate quantity of salt. After applying, a frequently used tool known as a refractometer is an easy way to check your salinity.

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Aeration and further filtering

Aeration is the process of adding extra oxygen to your water. This is accomplished by agitating the water’s surface, such as with waterfalls or aerators that produce bubbles.

Increased oxygen levels promote the growth of beneficial microorganisms. It also hastens the breakdown process. This means the bacteria can work more efficiently and quickly.

A filtration system might also help with the algae problem. On the one hand, water is maintained purely by filtering it; on the other hand, water movement provides enough oxygen. The pond water will be filtered mechanically and biologically by a filter apparatus. Mechanical filtering works by eliminating organic and inorganic filth particles as well as colorants.

The goal of biological filtration is to improve and accelerate the conversion of organic particles using microorganisms. An integrated biological filter boosts plant development by increasing the activity of micro life.

UV Light Sterilizers

An in-line UV sterilizer as part of your filtration system is one of the most efficient means of controlling single-celled algae like that which generates “green water.” In addition, it is a fantastic and non-invasive method of dealing with some forms of algae (and potentially hazardous bacteria) that may be easily added to your current pipework.

So, how exactly does UV radiation destroy algae? UV is a strong type of light radiation that efficiently penetrates the cell membrane on the algal cell’s exterior and reaches the algae’s sections where the DNA is located (nucleus and chloroplast). When this occurs, the UV disrupts the DNA to such an extent that the algal cell cannot reproduce.

If you only want to destroy free-floating algae, UV clarifiers are possible. Still, they are less effective (algae requires less powerful UV to be killed), so if you have a UV system, you may as well have one that also kills other organisms and bacteria. A 30-watt system should be enough for most ponds, but be sure the light is rated for the number of gallons you have.

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Another way of algae removal, particularly beneficial for bigger string algae, is simply scooping it out by hand! Most blanketweed on the pond top and most string algae on the sides may be removed with simple pond nets. They’re less successful at removing bottom algae and sludge, but they’re a low-cost choice on a tight budget for pond owners.

Pond vacuums are a great purchase for pond owners who find themselves cleaning their pond regularly if you want a faster choice with less physical labor. Compared to a pond net, vacuums can swiftly remove all string algae and blanketweed. They also provide excellent all-around pond cleaning by removing sludge and muck buildup, which can promote algae development.

These methods will not be efficient in removing most free-swimming green water algae. Thus they should be paired with a UV clarifier for optimal results. However, both procedures will produce considerably cleaner pond water and help prevent future algae development by eliminating additional nutrient sources like bottom muck.

Pond vacuums are a costly investment that may be overkill for extremely tiny ponds when a simple net would suffice. However, a decent pond vacuum is a beneficial long-term investment for algae management and general pond care for bigger ponds or owners who wish to make life simpler.

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Beneficial Bacteria

Beneficial bacteria are one of the most natural and safest ways to tackle algae. The bacteria will go for the causes of the algae rather than the algae itself. However, the procedure might take 30 to 60 days, and most pond owners are impatient. Aeration will aid in the proliferation of bacteria, resulting in a speedier outcome. 

Without aeration, the effects can be sluggish, and a greater volume of bacteria may be required. When it comes to helpful microorganisms, never underdose. Even though bacteria are beneficial to pond health, they may not always be sufficient to accomplish the task.

Aside from UV sterilizers, adding more beneficial bacteria is one of the most useful things you can do for your pond. This is especially true in the spring when your filter media is not as heavily loaded as it is in the summer.

Microbe-Lift PL is one of the most popular products for increasing bacteria populations, and they even have seasonal “blends” depending on your needs (and season).

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It’s generally a good idea to boost your bacteria now and then. Still, when it comes to algae, you may find yourself in a cycle where the algae die (naturally or through algaecides) and decompose on the bottom, causing high levels of nutrients like ammonia and nitrates. Those nutrients then fuel the next generation of algae.

Adding helpful bacteria allows nutrients to be digested before they become accessible to other algae, effectively limiting future algal development.

Koi Clay

Koi clay is one of those koi pond additives that can only benefit. This “stuff” is a natural approach to supplement your system with various beneficial minerals, and koi appear to enjoy it. It has also been observed that it is quite efficient for inhibiting and killing string algae as a side effect.

It is a calcium bentonite clay that may temporarily fog your pond if introduced. However, it clears up in a day, has added many essential elements, and eliminated pollutants. In addition, Kentucky is supposed to produce many superb racing horses because they consume the grass that grows in the state’s exceptionally calcium-rich soil.

Similarly, Japan’s koi may be adored because they are bred in clay-rich ponds. There are several excellent koi clays on the market, but you want ones that do not remain foggy for long periods.

Barley Straw

You may choose to utilize barley at the start of the season. It is available in various forms, including bales, pellets, and extract (a liquid form). For maximum efficacy, lay your barley bales or pellets anywhere the water runs, such as beneath a waterfall or within a skimmer. The barley will degrade and emit a harmless chemical into the water, aiding in the prevention of algae growth.

It is available as raw barley straw or as an extract. This green water treatment might take up to 30 days to fully activate, and the outcomes can be hit or miss.

Some believe the barley straw works by breaking down and producing a toxin that inhibits algae development. In contrast, others believe that the breakdown process releases hydrogen peroxide, which provides an unfavorable environment for algal growth.

It is crucial to note that while it slows algae growth, it does not destroy existing algae and should not be used as an algaecide. This treatment is more effective on free-floating algae than string algae and is most commonly used in the spring.

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Algae Treatment Chemicals


Most algaecides fall into one of three categories: those based on potassium permanganate, those based on copper, and those based on simazine. Simazine is a popular algaecide. This chemical operates by interfering with the photosynthetic process and destroying the algae.

This chemical should be handled with caution since it might hurt or hinder the growth of your aquatic plants (as they use photosynthesis, too).

Permanganate of potassium

Caution should be exercised while using potassium permanganate. It is not only used to eliminate parasites like costia, but it will also kill algae. However, you must monitor the pond after adding it. To begin, the dose should be roughly 1 teaspoon per 1000 gallons, but you may end up adding more or performing additional treatments depending on your needs and the number of algae you have.

You will need to increase your pond aeration since many of your fish will rise to the surface and pant for air. Continue the treatment for roughly 8 hours, and make sure the treated pond water does not pass through your filter media since this will kill your beneficial bacteria.

Potassium permanganate will be depleted as it kills algae and parasites, so you won’t need to do a large water change as you would if you added a lot of salt. However, vacuuming the bottom of your pond once the therapy is finished and your fish are not displaying indications of stress would be beneficial.

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Algae Fix

Algae repair belongs to the “copper-based” group. Most copper-based algaecides are chelated copper (which lasts longer than other forms). Because it inhibits algal cell metabolism, it can be successful as an algaecide; nevertheless, as with most treatments, there are certain dangers to be aware of.

Water clover and other vascular plants will be unaffected by the copper, but other plants that get nourished from the water may be. Another factor to consider is copper’s impact on invertebrate creatures such as snails and crayfish. Copper-based therapies will injure or kill most invertebrates since their hemolymph (blood) is copper-based.

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Green clean

One of the more recent solutions on the market, “Green Clean,” destroys algae by oxidation and achieves speedy effects. There is no residual, and it is not dependent on copper. Even though it is marketed as a “wide spectrum” algaecide, users have noted that it is best for string algal and not so well for green water (free-floating) algae.

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Accu Clearing

This treatment belongs to the group of treatments that cause green water algae to flocculate (suspended materials form small clusters and sink to the bottom). The idea is that your filter will manage the remainder; however, if you don’t clean the bottom afterward, it’s a good idea to boost your beneficial bacteria populations to handle the additional nutrient fallout from algae death.

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Every pond owner will have to deal with algae at some point. People typically battle with it in the spring as temperatures increase, but thankfully, there are many go-to treatments.

Some are broad-spectrum, while others are specific to one type of algae or another. The best option for you is one that suits your specific requirements. Several algaecide agents are on the market now, and many pond owners can vouch for their usefulness.

However, if you are overwhelmed by algae and unsure where to begin, try some of the remedies listed under “Prevention” in this article before using pesticides. You may have to resort to chemical treatments, and if you do, make sure to combine them with some of the preventative steps discussed, or you may find yourself in the same predicament again soon long.

Last update on 2022-09-11 / Affiliate links / Images from Amazon Product Advertising API