How To Remove Algae From A Pond Without Harming Fish

    How to remove algae from a pond without harming fish

    Algae may feel like a fact of pond life. It’s very common in outdoor ponds. Yet heavy growth can negatively impact our experience of owning a pond. So how do you remove algae from pond without hurting/harming fish?

    There are many ways to reduce algae levels in a pond. All of them involve targeting the excess nutrients and light that fuels algae growth. The more of them you take on, the more control you will have over algae.

    What Types Of Algae Grow Can Grow In A Pond?

    Pond algae

    This video does a great job of breaking down the different types of algae present in ponds:

    YouTube player

    Here are 4 types of pond algae you should know about:

    Green Water Algae

    The most common type of pond algae is green water algae. Unlike the others, green water happens due to free-floating unicellular organisms.

    It is one of the hardest algae blooms to control since it isn’t attached to a hard surface. Even if you do massive water changes green water algae can discolor your water again in just a few days.

    Hair Algae

    Hair algae growth is often seen on hard surfaces like rocks, gravel, and the liner of your pond. It can also grow tangled mats along the pond surface if it grows out of control.

    Hair algae grows slower than the other types. Plus many pond fish, including goldfish and koi, enjoy eating it.

    Green Slime Agae

    Green slime algae is similar to hair algae except it grows in a short coating. The lack of long strands makes it harder for fish to eat. Green slime algae can easily coat every portion of the surface area exposed to the sun.

    Green Algae Film

    Last, you may come across a green algae film along the surface. This film may have bubbles caught in it. And it usually does not smell very good. These are blue-green algae blooms caused by cyanobacteria.

    Why Should Algae Be Removed From A Pond?

    Algae has no way of harming fish. It can even be a food source for them. So why is pond algae control important?

    Algae growth is an aesthetic issue. A pond that looks like pea soup is not at all fun to look at. Any fish swimming will be barely visible once they leave the surface. You may only get to see them when they rise to the surface for feeding.

    Decorations submerged in fish ponds can become coated in green hair or slime algae. Hair algae at the surface can shelter mosquito larvae from your fish.

    Rocks also no longer look attractive when covered in muck. The entire pond ends up looking unkempt. It’s been transformed from a calming water feature to a place few people want to visit.

    If you want to have a beautiful pond, such as a koi pond, it’s a good idea to remove algae from it every now and then.

    How To Remove Algae From A Pond Without Harming Fish

    How to get rid of algae in a pond
    Grass carp (Ctenopharyngodon idella)

    Removing algae from your pond water without harming fish need not take harsh chemicals or a lot of time. Here are two ways to quickly accomplish your goal!

    Supplies Needed

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    Barley Straw Removal Method (Naturally Removes Algae)

    If you are looking for all-natural algae control methods, barley straw is by far one of the best options. It comes in large bales that are left to float in a pond. While not very attractive, barley straw bales do a good (if slow) job of killing algae.

    Barley straw should be used only in the summer. Warm water temperatures and sunlight are necessary for the product to impact an algae bloom.

    They work via decomposition. As beneficial bacteria consume the barley straw it breaks down. This releases humic acids into the pond water. Which are then broken down further by sunlight into hydrogen peroxide, a well-known cleaning agent.

    Peroxide levels remain low enough not to harm fish and more complex life. But high enough to target the simple cells of algae, especially green water algae.

    The only step to using barley straw is to toss a straw bale into your pond and allow it to float. Over the course of 1-2 weeks, you will see a reduction in algae growth.

    Barley straw is best for very large ponds, where manual cleaning is not feasible.

    UV Clarifier Removal Method

    Running a UV light device is one of the best ways to remove algae from a pond without harming fish. It is also the easiest to set up and operate.

    UV clarifiers use ultraviolet light to kill algae cells that flow through them. They are only effective on green water algae, not the types that grow on hard surfaces.

    UV light has no impact on fish and other aquatic life. It’s also free of chemical treatments and can be run for as long as you wish.

    Step 1: Turn off your filter, if you are setting up an in-line UV clarifier. These process the water before or after it leaves the filter. There are also UV clarifiers that run on their own, free of the pond filter. Both models are equally effective at killing garden pond algae.

    Models that run separately from the pond filter usually have a sponge block on the intake to keep debris from clogging them.

    Step 2: Attach the UV clarifier to an intake and outtake line. The exact instructions will depend on the model of clarifier you choose.

    Step 3: Turn the filter back on, if running an in-line clarifier. Make sure any sponge filters are attached to the intake of a free-running clarifier. And plug the unit in.

    How To Clean Algae Off Rocks In A Pond Without Harming Fish

    How to get algae off rocks in a pond
    Pumpkinseed Sunfish (Lepomis gibbosus)

    There are two main ways to manually remove algae from rocks without harming fish. The first is to physically remove the rocks and hand-clean them. And the second is to power-wash rocks.

    Hand scrubbing is best when you have just a few, easily removed rocks. Power washing is the better choice when your garden pond has several rocks that are too large to be removed easily.

    Hand Scrubbing Pond Rocks

    Hand scrubbing rocks is as simple as it sounds. For smaller ponds, all you need to do is to remove any rocks coated in algae. A stiff bristled brush will allow you to strip away the green coating. And then gently place the rocks back into your pond. Or redecorate, if you feel like shifting the arrangement!

    Power Washing Pond Rocks

    Step 1: Turn off any filters, clarifiers, and other equipment. If left to run while the pond is draining, they may run out of water. Pond pumps will grow too hot and burn out if run without water for even a short period.

    Step 2: We first need to drain the pond to access the rocks. This can be done using a siphon hose.

    Except that most of the time, gravity-powered siphon hoses won’t do the job. If you use one for your inside fish tank then you know they need a height differential to operate.

    Instead, we need a siphon hose that runs off of water pressure, like the Python No-Spill system. You simply attach one end of the siphon hose to an outdoor spigot. And the other end runs directly into your pond, draining it.

    Send some of the pond water into a bucket for step 3.

    Step 3: Once the pond is partially drained, catch any fish and invertebrates living in it. Place them into the bucket of pond water captured from step 1. Any floating plants and potted pond plants can be placed in a bucket as well.

    Aquatic plants need to be protected when the pond is drained. You can also wrap them in wet newspaper and place them in the shade or in a bucket.

    Dry air and direct sun exposure are deadly to aquatic plants. They will die if allowed to dry out. So check on them periodically while cleaning your pond and re-wet them as needed.

    Step 4: Remove the siphon hose from the outdoor spigot and attach your pressure washer. Set the outflow so that it’s powerful enough to strip away any algae growth.

    But not so strong that it damages the pond lining. Most pressure washers won’t tear a pond lining but old or cheap liners may be susceptible.

    Algae will come free from garden pond rocks easily with a pressure washer. You can rent them from most home improvement stores for a day of pond maintenance.

    Step 5: Set up your siphon hose again and suck up the remaining pond water. This water now has all of the rock and liner algae growth in it. If you refill the pond with these algae remnants it will simply re-attach itself.

    What Eats Algae In A Pond?

    Get rid of algae in pond with fish
    Goldfish (Carassius auratus)

    Most aquarium algae eaters don’t do well in unheated ponds. Fortunately, there is a wide range of outdoor pond algae eaters you can choose from. These fish and invertebrates tend to stick to the bottom, where algae grows the thickest.


    Believe it or not, some of the most common pond inhabitants make great algae eaters. Goldfish eat huge amounts of algae. But they don’t have the flat, sucking mouths of most true algae eaters.

    Goldfish stick to string algae and other thickly growing forms. You will need another fish or invertebrate for slime algae films.

    Scientific Name: Carassius auratus
    Length: 12 to 16 inches
    Types of Algae Eaten: Hair

    Apple Snail

    Snails of all kinds are fantastic algae eaters. A group of them can eliminate aquatic algae from a small pond (except green pond water). The key is to choose a snail that does well in unheated water. And one that won’t be eaten by your fish.

    As prolific and hard-working as ramshorn snails are they are perfect snacks for goldfish and koi. Full-grown apple snailsare a better choice. The smaller the fish and the larger your snail, the better.

    Scientific Name: Family Ampullariidae
    Length: 3 to 5 inches
    Types of Algae Eaten: Hair, Slime

    Dojo Loach

    Dojo loaches are often found in aquarium stores. They are often sold as weather loaches because they are sensitive to changes in atmospheric pressure. When storms are approaching they become wildly active.

    Weather loaches will eat anything they find along the bottom. They aren’t as dedicated to eating algae as a plecostomus. But they will eat strands of green algae along with leftover flakes and pellets.

    Scientific Name: Misgurnus anguillicaudatus
    Length: 8 to 12 inches
    Types of Algae Eaten: Hair, Slime

    Bullfrog Tadpoles

    This one might surprise you, but tadpoles are excellent algae eaters. Frogs are carnivorous but tadpoles are omnivores. They will eat anything they can find to fuel their metamorphosis. Dead algae, live algae, dead fish, and even each other.

    Bullfrog tadpoles are large enough that only the biggest pond fish can eat them. Once they transform into frogs with limbs, they may decide to leave, however. Be certain to check your local regulations.

    Non-native bullfrogs are big predators. They are an invasive species that can devastate local insect and amphibian populations. Only keep bullfrog tadpoles if they are already found in your region.

    Scientific Name: Lithobates catesbeianus
    Length: 2 to 4 inches
    Types of Algae Eaten: Hair, Slime


    How Long Does It Take To Remove Algae From A Pond?

    It generally takes at least a week, if not a few weeks to remove algae from a pond. It depends on the source of your algae (light, nutrient imbalances, etc). As well as how many corrective steps you take. That said, you will always have some algae. Indeed, a pond with no algae is a sign of a sterile, dead environment.

    How To Prevent Algae From Growing Back In A Pond

    Ninespine stickleback (Pungitius pungitius)

    Now we understand how to remove algae from pond without harming fish. But how can we keep algae from being a huge problem again?

    Reduce Sun Exposure

    First, think about ways to reduce incoming sunshine. Direct sun fuels algae growth like nothing else. So if you haven’t already built your pond, factor hours of sun per day into your plans.

    An ideally lit pond should have no more than a few hours of direct sun per day. The indirect sun diffused through trees is a different story. If you live in a cloudy part of the world, that also changes how you approach sun exposure.

    Direct sun also heats up a pond quickly. Sudden temperature shifts are very stressful for goldfish and other aquatic life.

    Adding Aquatic Plants

    A healthy pond in nature has a large diversity of aquatic plants living within it. These plants all compete with each other, as well as algae and other aquatic life. An outdoor pond should look like a water garden full of lush growth.

    If your pond has few to no plants, then algae problems are only natural. Something has to make use of the carbon dioxide and nutrients that fish release.

    Floating Plants For Ponds

    Free-floating water plants are great natural algae control methods. They also add beauty and interest to any garden pond. Even a small pond has room for a few floaters!

    Floating plants grow fast and need next to no care. They get all of the light and carbon dioxide they need from the surface. Fish ponds are high in nutrients due to the inhabitants.

    Floating plants also shape a pond’s ecosystem by reducing the amount of sunlight entering your water. The less light that hits garden ponds the more algae control you have.

    Normally, algae has little competition for light and excess nutrients. Floating plants also absorb phosphates, nitrate, ammonia, and other fertilizers. Helping you get algae fully under control!

    There are dozens of good floating plant species to add. It all depends on which species are the most interesting for you. Small pond plants like duckweed are helpful. Except that duckweed will cover the entire surface of a pond, hiding your fish.

    Red root floater (Phyllanthus fluitans) is easier to manage while still creating the same swamp aesthetic. As are water lettuce (Pistia sp.) and water hyacinth (Pontederia crassipes – illegal in some US states).

    Rooted Plants As Algae Control Methods

    Submerged aquatic plants don’t directly compete with algae for light. But they do absorb nutrients in the same way as floating plants. The fewer nutrients there are, the less there is for pond algae growth.

    Elodea are popular, fast-growing pond plants. Goldfish and koi are known to eat them. But if you start out with large enough bunches they will grow faster than your fish can consume them.

    Hornwort (Ceratophyllum sp.) is less appetizing and incredibly hardy. It can even be left to float, though it creates untidy tangles at the surface.

    Water lilies (Nymphaea sp.) are also popular for algae control and beauty. They aren’t technically floating plants. But the broad surface leaves of lilies still reduce sunlight reaching the water column. Plus the flowers come in all sorts of beautiful colors!

    Reduce Fish Feeding

    Another way to control algae is to ensure you aren’t overfeeding. Pond water saturated with nutrients is an ideal habitat for algae. Especially if the water garden is getting a lot of sun and there are no aquatic plants.

    Only feed as much as your fish will eat in a few minutes. And it’s always okay to skip a day, here and there. Full-grown goldfish and koi only need to be fed once or twice per day.

    No matter how much they beg, don’t let them train you into feeding them several times a day. Your pond water will be cleaner for it. And you will have to do less maintenance Leaving you more time to enjoy your water garden!

    You can also use high-quality fish food with more digestible ingredients. Choose brands that are low in phosphorus, an important algae nutrient.

    And make sure you keep up with the seasons. In temperate and cold climates, you should shift to a spring and fall blend with the seasons.

    Goldfish and koi still need food when the water temperatures are cool enough to slow their metabolism. But not so cold as to send them into torpor.

    Spring and fall fish food formulas are more digestible, ensuring fewer nutrients go to waste. And they pass through the system of your fish faster so they don’t go to sleep with a full gut.

    Keep Fewer Fish

    Lastly, remember that too many fish means more pets creating excess fish waste. Fish poop is full of nutrients that algae can use. Your filter and beneficial bacteria will break down most of it.

    But excess nutrients from any source need to be reduced if you want to get algae under control. Fish also excrete ammonia directly into the water column, which is the perfect fertilizer for algae and plants.


    Controlling algae in a pond can be difficult for beginners to do. It involves managing several nutrient sources at the same time. As well as the amount of light coming in.

    Fortunately, there are many corrective steps you can take to achieve a better balance in your water garden!

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