pond water filled with algae

From backyard koi ponds to sprawling farm ponds, every pond will eventually fall victim to an algae outbreak. Whether it’s turning the water green or coating the surface in a stringy, slimy mess, algae is always an unwelcome sight for a pond caretaker. The faster that algae are removed the better!

For ponds with fish, the question of how to kill the algae is complicated by the need to leave the fish unharmed. Thankfully, there are products available that will get rid of pond algae without taking the fish down with it. We’ve put together reviews of the 10 best pond algaecides and algae killers that are also safe for fish when used as directed. Hopefully, comparing these products will help you find the right one to get your pond back to being clean and clear.

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A Quick Comparison of Our Favorites in 2024

The 10 Best Fish-Safe Pond Algaecides & Algae Killers

1. API Pond Algaefix – Best Overall

API Pond Algaefix

Which Algae It Kills:Green water algae, string algae, blanketweed
Active Ingredients:Dimethyliminio ethylene dichloride, ethoxylate – 4.5%
Safe for:Live plants and pond fish

Our choice for the best overall pond algaecide and algae killer is API Pond Algaefix. A fast-acting liquid algaecide, this API product is effective against the most common types of pond algae. While this algae killer is safe for fish, it’s essential that you follow the directions for use to make sure. In particular, make sure you know the total volume of your pond to avoid adding too much product. When using any chemical algae-killer, be certain your pond continues to get enough oxygen, usually by using a pond aeration system. Depleted oxygen levels due to mass algae die-off are dangerous to fish. API Pond Algaefix can clear an existing algae outbreak, as well as be used to prevent future outbreaks. A major selling point of this product is how quickly it works, but you should expect a slower kill if you have a very heavy algae infestation.

  • Kills target algae quickly and effectively
  • Helps prevent future outbreaks
  • Reasonably priced
  • Not safe for crustaceans like shrimp or crabs
  • Less effective for heavy algae infestation
  • Doesn’t kill every type of algae

2. Tetra Algae Control Pond Block – Best Value

Tetra Algae Control Pond Block

Which Algae It Kills:Green algae, string algae
Active Ingredients:Copper sulfate, diuron
Safe for:Fish

Although its use is limited to ornamental or backyard ponds, one of the best algaecides for the money is the Tetra Pond Block. This product is very inexpensive compared to other products on this list and easy-to-use. The downside is this product can’t be used with live plants and only in backyard ponds rather than naturally occurring ponds. Despite its strict limitations, the Pond Block is effective when it comes to clearing algae from ornamental ponds or fountains. The slow-dissolving blocks continue to control algae for about 30 days, yet another reason why this algae killer offers good value. As always, follow directions carefully and make sure your pond is aerated.

  • Inexpensive
  • Long-lasting
  • Effective in ornamental ponds and fountains
  • Use limited to ornamental ponds
  • Not safe for live plants

3. Green Clean Granular Algaecide – Premium Choice

Green Clean Granular Algaecide

Which Algae It Kills:Labeled for string algae, also kills blue, green algae
Active Ingredients:Sodium carbonate peroxyhydrate
Safe for:Fish and plants

One of the fastest-working algaecides, Green Clean Granular Algaecide begins to kill algae on contact. While it’s most effective against string algae, this product will kill other algae as well. Green Clean works so quickly you’ll have to be ready to clean dead algae out of your pond quickly before it sinks and starts to decompose. High quantities of decomposing algae can reduce the oxygen levels in your pond, putting fish in danger. Be extra diligent about keeping your pond aerated. This product is a good choice for anyone concerned about adding harsh chemicals to their pond. Green Clean kills algae by breaking down the individual cells on contact.

  • Kills algae on contact
  • Easy to apply
  • Doesn’t include harsh chemicals
  • Dead algae must be cleaned up quickly
  • Formulated for string algae only, but does kill others

4. Microbe-Lift Algaway 5.4 Algaecide

Microbe-Lift Algaway 5.4 Algaecide

Which Algae It Kills:Green water algae, string or hair algae, blanketweed
Active Ingredients:Polyethylene, ethylene dichloride
Safe for:Fish and plants

Microbe-Lift Algaway is a liquid algaecide designed to kill the major types of pond algae. This product is safe for most pond fish and ornamental plants when applied as directed. While safe for koi and goldfish ponds, it’s important to be sure your pond is popularly aerated before applying Algaway. In addition, if you have a heavy infestation, it’s recommended to rake and remove some of the algae before using the algaecide to avoid any danger to your fish as the decomposing algae decrease water oxygen levels. This product can take longer to kill algae than others and may require multiple treatments.

  • Kills all major algae
  • Easy to use
  • Can be dangerous to fish if used incorrectly

5. Tetrapond Algae Control Water Treatment

Tetrapond Algae Control Water Treatment

Which Algae It Kills:Green water algae, string or hair algae, blanketweed
Active Ingredients:Polyethylene, ethylene dichloride
Safe for:Fish and plants

Another product from Tetrapond, the Algae Control Water Treatment is safe to use with plants and can be used to treat large ponds, as well as the backyard version. This product is sold as a concentrated liquid, which must be diluted before use. Because it’s concentrated, one container of this product goes a long way, making it a good choice for larger ponds. Algae Control kills by breaking down algae cells but doesn’t work as quickly as Green Clean. It may take a full 3 days before you know if another treatment is needed. With this product, you’ll need to ensure that not only are you using the right amount of product for your pond but that it’s diluted correctly as well.

  • Kills all types of major pond algae
  • Cost-effective for large ponds
  • Must be diluted

6. Aquascape EcoBlast Granular Algaecide

Aquascape EcoBlast Granular Algaecide

Which Algae It Kills:Green water algae, string or hair algae
Active Ingredients:Sodium carbonate peroxyhydrate
Safe for:Fish and plants

Aquascape EcoBlast is another product that kills algae by breaking down algae cells on contact. A granular rather than liquid treatment, this product works best as a quick spot treatment for algae. Unlike some of the other algaecides, Aquascape doesn’t work as a preventative treatment. While it kills algae quickly, it sometimes returns just as quickly. The company makes other products designed to maintain pond health and prevent algae which you might need to use in addition to this product. EcoBlast works at many different water temperatures and pH levels. With heavy string algae infestations, you might need to rake and remove some algae for this product to work most effectively.

  • Can be used in a wide range of water temperatures and pH
  • Easy to apply
  • Works quickly
  • Not good at keeping algae from returning
  • Might be less effective with heavy string algae outbreaks

7. Crystal Clear Algae D-solv

Crystal Clear Algae D-solv

Which Algae It Kills:Green water algae, string or hair algae, blanketweed
Active Ingredients:Polyethylene, ethylene dichloride
Safe for:Fish, plants, wildlife

Crystal Clear Algae D-solv is a quick-acting algae-killer safe for pond plants and fish when used correctly. This product can be used whenever algae outbreaks happen but not for maintaining pond health afterward. Algae D-solve is also one of the few algaecides to specifically say it’s safe for any wildlife that may drink or swim in your pond. As with all chemical algae killers, use only the recommended amount to ensure your fish and plants stay safe. Because it kills algae so quickly, Algae D-Solv can cause pond filters to become clogged with dead algae so make sure you check and clean them often until your pond is clear.

  • Fast-acting
  • Safe for wildlife as well as fish and plants
  • May need to repeat treatment frequently

8. API Pondcare Microbial Algae Clean Green Water Biological Inhibitor

API Pondcare Microbial Algae Clean Green Water Biological Inhibitor

Which Algae It Kills:Green water algae
Active Ingredients:Bacillus bacteria
Safe for:Fish and plants

Chemical algaecides may work the fastest but for those worried about adding any kind of chemical to their ponds, API Pondcare offers an alternative. This product uses bacteria that feed on green water algae to clear ponds safely and without adding any chemicals to the aqua environment. Because it’s not a chemical, it may take longer to work and must be re-added to a pond consistently to keep algae under control. In addition, Pondcare is only effective against green water, not other types of algae. The main appeal of this product is that it’s a natural method of algae control, completely safe for fish and plants.

  • No chemicals
  • Won’t upset the pond ecosystem
  • Only works on green algae

9. Tetrapond Barley and Peat Extract Clear Water Treatment

Tetrapond Barley and Peat Extract Clear Water Treatment

Which Algae It Kills:Green water algae, helps prevent string algae
Active Ingredients:Barley straw extract, peat extract
Safe for:Fish and plants

Bales of barley straw are the original all-natural algae killer. It’s believed that the barley straw releases a chemical as it decomposes that stops algae from growing. To get all the benefits of barley straw without the hassle and mess of hauling and scattering bales, try TetraPond Barley and Peat Extract. If you are dedicated to natural methods of algaecide but have a heavy algae infestation, you may need to physically remove much of the algae before using this product. Barley and Peat Extract is more effective at keeping pond water clear and preventing outbreaks rather than killing off algae. When used regularly, it will kill existing algae but much more slowly than chemical algaecides.

  • All-natural
  • Safe for all fish, wildlife, plants
  • Helps keep pond water healthy in addition to algae-free
  • Works slowly
  • Not effective on its own for heavy algae outbreaks

10. Tetra GreenFree UV Pond Clarifier

Tetra GreenFree UV Pond Clarifier

Which Algae It Kills:Green water algae
Active Ingredients:Ultraviolet light
Safe for:Fish and plants

Our final algae-killing product is the most unique and has the highest upfront cost but is an effective, completely natural way to kill green water algae. The GreenFree UV Pond Clarifier is easy to install and blasts green water algae with ultraviolet light, destroying them. This product doesn’t work quickly, taking up to 8 days to clear up your pond water. However, it’s completely safe for fish and plants. Once the initial algae bloom is clear, keep the UV Pond Clarifier running to prevent further infestation. This product is best for small backyard ponds and only works on green water algae.

  • Easy to install
  • All-natural algae killer
  • Only works on green water algae
  • Expensive upfront cost

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Buyer’s Guide: Selecting the Best Fish-Safe Pond Algaecide & Algae Killer

When you’re deciding on a pond algaecide, here are a few things to consider to help you make the right choice for your pond.

pond water with algae
Image Credit: analogicus, Pixabay

Natural vs Chemical Algae Control

The answer to this question usually comes down to your comfort level with adding chemicals to your pond as well as how quickly you want your algae gone. Chemical algaecides work dramatically faster than natural methods so if you’re in a hurry, go for one of those. If you are willing to tolerate a slower kill if it means no worry about your pond ecosystem being harmed, pick a natural algae killer like the UV purifier or barley straw extract.

Does Your Pond Have a Source Of Oxygen?

All chemical algaecides, even if they’re labeled fish-safe, pose a risk of killing fish from depleted pond oxygen. Dead algae lower the oxygen levels in a pond as it decomposes, posing a threat to the health of both fish and pond plants. If you plan to use a chemical algaecide, make sure you have a way to aerate your pond. If not, play it safe and choose a natural alternative.

Is Your Pond Fully Contained?

Most chemical algaecides can only be used in fully contained ponds with no outflow into the environment via streams or drainage pipes. If that’s not your pond, opt for a natural algae control method.

Final Thoughts

Now that you’ve read the reviews of our top 10 pond algaecides, it’s time to get to work clearing out your pond! Our Best Overall pick, the API Pond Algaefix, is both effective and cost-effective when it comes to dealing with an algae infestation. To keep a backyard pond algae-free, consider our Best Value pick, the Tetra Algae Control Pond Block, an inexpensive, long-lasting choice of algaecide. Whatever type of algaecide you choose, follow all directions carefully to make sure your fish stay safe.

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Featured Image Credit: Stephen Marc, Pixabay