Pond surrounded by trees

Though you might not enjoy the aesthetic, some algae in your pond are normal. Algae thrive off nitrates in koi waste and are an essential part of your pond’s ecosystem. These plants add oxygen through photosynthesis and make a healthy snack for koi.

However, increased waste, water temperatures, and sunlight can cause algae to overpopulate in the summer. An abundance of algae will consume more oxygen than it creates, which is a problem because your koi require more oxygenated pond water in the summer.

Fortunately, many methods exist to reduce pond algae. If your overgrowth is mild, natural solutions are safe and effective at removal and prevention. If the problem is moderate or severe, your pond might require mechanical or chemical treatments before you can employ preventative measures.

What Is Algae?

Planktonic (green water algae) and filamentous (string algae) are the two main types of algae that affect koi ponds. Sunlight exposure coupled with excess nutrients in the water causes planktonic algae. This condition is common in new ponds or ponds with inadequate filtration.    

Filamentous algae thrive when filtration is weak or beneficial bacteria are insufficient for the pond size. They grow from the bottom of the pond, break into pieces and float to the surface. Filamentous algae clog filters and water features and produce blooms that can double their weight in 24 hours, making them difficult to remove.

Removing Algae: Natural Methods

Natural methods of removing and preventing algae overgrowth are every pond owner’s first line of defense. These methods are safe for pond inhabitants, and many are already part of an effective pond maintenance routine.

Manual Removal

If your pond has large algae blooms, the best preliminary step is to scoop, rake, or vacuum the bulk of them before treating the rest. This can be a time-consuming process and is not a long-term solution (the algae will grow back). However, it can create a more manageable situation for you to work with as you address the underlying causes of the algae growth.


Barley straw is an ecologically friendly option that requires minimal effort–you just toss it in the pond. Barley works slowly but effectively clears small amounts of algae, and prevents regrowth. Barley converts to algae-killing hydrogen peroxide when it decomposes but steadies at safe levels for fish.

You might consider purchasing barley extract if you cannot wait a month or two for the decomposition process to yield results. Barley extract works faster but can be dangerous for your koi if not dosed precisely.

Pond Plants

Pond plants control algae by depriving it of the nutrients it needs. To effectively control algae, you must cover at least 60% of your pond’s surface with plants. This is perhaps the simplest long-term solution to keeping water clean and clear.

Floating plants like lilies and lotus reduce algae’s access to life-sustaining sunlight. Submerged plants such as hornwort release oxygen to help counter the algae’s effects.

Pond Algae Eaters

Koi are conservative algae-eaters, so one option is to add aquatic companions to the pond that consume it more voraciously.

The Chinese Hi Fin Banded Shark is actually not a shark but a species of fish, and it is a fantastic bottom feeder that will help consume the algae at its source.

The Japanese Trapdoor Snail is considered the best algae-eating snail for a koi pond. Though they are slow eaters, these snails will meticulously nibble at pond algae. Plus, they won’t harm your other live plants.

Otocinclus Catfish are a small catfish breed that many pond owners consider among the best algae eaters. These fish will consume all algae, from slimy brown to newly growing.

The Siamese Algae-Eater lives up to its name. These fish are great for warm climates and will work day and night to gobble as many algae as possible. They do well with koi and goldfish but cannot survive cold temperatures.

Avoid Overfeeding

You should feed koi the amount of food they can consume in about five minutes. Leftover food will drift to the bottom of the pond and rot, which is a catalyst for algae growth. It is also important to note that feeding requirements change seasonally, so the amount of food koi consume in five minutes will be less in the spring and fall than in the summer.

High-quality fish food available at suppliers like Blue Ridge Koi & Goldfish will help to alleviate this problem. Lower-end koi food often contains filler ingredients that the fish cannot process, leading to excess waste production that facilitates algae growth.

Structural Changes

Water runoff typically contains nutrients that algae can feed on to grow. One way to combat this is to build a rim around the edge of your pond to block it. If you are building a pond, placing it at a high point in your yard will help avoid this problem.

If you consistently have problems with algae in the summer, you might consider adding depth to your pond. A deeper pond stays cooler in the warm summer months, making it harder for algae to grow. Ponds with graduated bottoms tend to do best at preventing algae growth, with the ideal parameters being 40% deep, 30% intermediate, and 30% shallow.

Improve Water Quality

Weekly water changes introduce clean water to the pond and reduce excess nutrients. The general recommendation is to replace between 10% and 20% of the water each week, not including evaporated water. Do not change the water more than once a week as this can stress the fish and cause an imbalance in water parameters.

If your pond has a filtration system installed but algae are still growing, you may need to clean the filter inside the system more often. Follow the manufacturer’s instructions to clean your filter at least once each month to prevent algae from forming.

Beneficial Bacteria

Biological augmentation can improve pond water quality by adding minerals, enzymes, and microbes. Stimulating these substances in the water will reduce nutrients such as phosphorus that boost algae growth.

Removing Algae: Mechanical and Chemical Treatments

Used in combination with natural solutions, mechanical equipment and chemical treatments can help rid your pond of algae overgrowth and keep it under control in the future.

UV Pond Clarifiers

Ultraviolet light keeps planktonic algae from spreading and destroys it as it grows (it will not work on filamentous, or string, algae). Once killed, the plant particles clump together and the filter removes them. UV clarifiers can attach to your pond pump or filter.

When selecting a UV clarifier, consider your pond and pump size. A pump that is too large won’t expose algae to UV radiation long enough to kill it. Replace the UV light bulbs yearly to maintain effectiveness. Additionally, overexposure to UV light can harm beneficial plants and bacteria in your pond. You will need to know your pond’s size and volume to determine the appropriate wattage.

Pond Dye

Color the water with a pond dye to reduce the sunlight it absorbs. There are several dyes, ordinarily blue, which stop sunlight from reaching the bottom and prevent the formation of algae. Pond dyes can also help keep your fish hidden from predators that are more active during summer.


One of the leading causes of algae blooms is the lack of water movement. Purchase a fine bubble aerator and install it in the deepest section of your pond, which will help add oxygen to the water and keep it moving to produce a healthier environment in the pond and prevent algae growth.


Pond algaecides work by attacking the algae cells and destroying the cell wall. This effectively kills and destroys the algae, making such algaecides a highly effective remedy.

However, oxygen levels in the pond will become lower as the algae die off. An abrupt and further drop in your pond oxygen can be dangerous for your fish. Use algaecides as a last resort when you can’t clean your water and remove your algae any other way or the situation is complex enough that the benefits outweigh the risks.

Selecting the best algaecide for your pond will depend on the types of pond algae you are looking to treat. For fine planktonic algae that cause green water, Algae D-Solv or other liquid chemicals are a great option. If you’re fighting filamentous (string) algae, a granular form such as AlgaeOff is more effective since it makes better contact with the algae.

Are you ready to make your pond environment the best it can be? Shop our full selection of koi and pond goldfish today.

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