Can Algaecide in a Pond Kill Fish or Plants?

As a pond owner, you may have to deal with algae. Algaecides are chemical products that kill algae in your pond.

They contain copper sulfate, potassium permanganate, and sodium carbonate. Algaecides are available at pet stores or garden centers and are frequently used in ponds to help control algae blooms.

You can buy Algaecide as a liquid, granule, or powder product. The most effective way to kill algae is to treat it with the right product at the correct dosage. It is important to know the right dosages for ponds, as well as how to apply an algaecide and when to do so.

What is Algaecide?

Algaecides are chemical products used to kill algae in ponds. If you have a pond and green water, an algaecide may help clear the green water to restore the clarity of your pond.

Most Algaecides products can be used in ponds with fish and plants.

Can Algaecide kill my Fish?

Algaecide can kill fish. However, if you use it correctly, the chances are slim that you will end up killing your fish.

Algaecides work by destroying the cell walls of algae. This causes them to separate from one another and rise to the surface where they can be scooped up or skimmed off of the water’s surface.

When using an algaecide, it is important to follow instructions carefully, as some algaecides are quite toxic and can be dangerous if they get in the water. Some algaecides are recommended for use on specific types of algae only and should never be mixed with different types of algaecide, or with other chemicals.

To minimize the risk of killing your fish, follow these steps: First, test the water’s pH to make sure it is between 6.8 and 7.2, and that the water is free of ammonia, nitrite, and chlorine.

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Add algaecide gradually to your pond at a rate of 1/2 cup every 2-3 hours over a 24 hour period only if you have thick mats of algae growing on the surface. Never use Algaecide in an outdoor pond when the temperature is below 55 degrees Fahrenheit.

In colder conditions, algaecide will not be as effective and your fish could die if they are exposed to it. Continue adding algaecide for a period of 2-3 days, then stop. After you stop adding algaecide, test the water’s pH and ammonia levels daily, for at least 2 weeks.

If the pH rises above 7.2, or ammonia levels increase to more than .25 ppm, you will need to continue treating with algae medication until these conditions no longer exist in your pond water.

With proper handling and use of algaecides, it is possible to effectively control algae growth without killing fish, however, it is always best to be cautious.

pond algaecide safe for fish

Here are 3 algaecide products that can be safe for your fish, but may not be safe for your plants.

Aquascape Algaecide Treatment

  • Eliminates algae in just 24 hours
  • Safe for fish and plants
  • Ideal for ponds with beneficial insects or live stock

Safe for Fish & Plants: Aquascape Algaecide is safe to use around fish and plants. It contains no copper, so it won’t kill your pond critters. And while Aquascape Algaecide can kill algae, it won’t harm your fish (or you!) when used as directed.

GreenClean Granular Algaecide

Pond and water garden treatment design to prevent the growth of unwanted algae plants and kill existing algae. This algaecide product is safe for fish, aquatic animals, and plants when used as directed.

GreenClean Granular Algaecide can be used in ponds with live plants by using an application method that will minimize contact between the algaecide and plants.

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Applied Biochemists Cutrine-Plus Algaecide

This product is formulated to be safe for fish and plants.

It is 100% water-soluble, non-corrosive, biodegradable, and contains no cadmium or lead. It has been tested extensively by independent laboratories to ensure it meets or exceeds the EPA’s requirements for aquatic safety.

Experts recommend that you apply algaecide in the spring before algae begin to grow, and then again in the fall to control algae over-wintering in your pond.

Is algaecide safe for my plants?

Yes, algaecide can be safe for your plants. Plants react differently depending on the type of algaecide and the concentration that you apply to the pond water.

Algaecide must be used in accordance with the product label instructions.

Some algaecides are very safe and can be used at low concentrations, while others require higher concentrations.  Additionally, some products should always be used in conjunction with copper sulfate to prevent the algae from developing a resistance to them.

How can you remove algae from the pond without harming your fish?

Algae is a natural occurrence in ponds, and fish love it. While excess algae growth can be unsightly (and smelly), most times there is nothing to worry about. However, if your pond has extra heavy algae growth that might kill or suffocate the fish, there are steps you can take to eliminate it without harm to your fish.

Algae-eating fish

Some pond owners looking for algae-eater fish. However, you need to know a few things about these fish before you decide whether or not this is the best course of action for your particular situation.

Some types can survive in cold water and others are better suited for warmer ponds, some are better able to withstand poor water quality, and some need a certain amount of dissolved oxygen in the pond water. In other words, before you spend money on algae-eaters, think about your pond’s environment and decide what kind of algae-eater fish would thrive there.

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Diatomaceous Earth (DE)

Diatomaceous earth is made up of fossilized silica shells. It works by scraping away at the outer layer of any organism that comes in contact with it. While diatomaceous earth is generally considered safe for humans, pets, and wildlife, you should always wear protective gear when handling it and be cautious around treated ponds while animals are present.

Algaecides or herbicides

You might consider using an algaecide to kill the algae, as this may eliminate chemicals you do not want in your pond water. However, herbicides (weed killers) that are intended for use around ponds and aquatic environments may harm fish if used without extra care. This is because most herbicides kill or negatively affect nearly all plants and algae (even beneficial types) by killing their roots. So again it’s important to consider the type of algae growing in your pond before messing with chemical treatments on your own.


Biological treatment involves adding fertilizer to promote the healthy growth of bacteria and then aerating the water to encourage algae growth. Once the algae have grown big enough, it is harvested and disposed of.

While a biological approach may be better than algaecides or herbicides in terms of affecting only the specific types of algae you are trying to get rid of, it comes with its own set of problems, since it involves adding nutrients to encourage the growth of an organism that can be harmful to fish, and it still results in the removal of all types of algae.


Managing and Controlling Algae in Ponds