Identifying Common Types of Pond Algae

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This article will be your guide for identifying the good, the bad, and the ugly algae you may encounter.

If you already know what type of algae problem you have, you may want to read our guide on pond algae control & removal.

Where there is water there will be algae.

As a pond owner, it is important to be able to identify the different types of algae that can grow in your pond. When you can do this, you’ll be able to determine if the algae needs to be removed or if they are good for the ecosystem.

Different Types of Algae Commonly Found in Ponds

From complex macro-algae like giant sea kelp to simple single-cell planktonic algae, there are more than 150,000 known types of algae[1].

Luckily for pond owners, only a small fraction of these different types of algae grow in ponds!

For this article we complied the most common types of algae you are likely to find in your pond (including a few non-algae frequently mistaken as algae!)

The Worst Type of Algae: Blue-Green Algae

While many types of pond algae are unwanted because they look unsightly, our experts agree that Blue-Green Algae (BGA) is the worst type of algae to have in your pond  because of its ability to cause Harmful Algae Blooms (HABs) & toxic algae incidents around the world.

Harmful Algal Blooms and Cyanobacteria Research

Ironically, blue-green algae are not algae at all!

While both photosynthesize and are similar in appearance, the term blue-green algae actually refers to a class of bacteria named cyanobacteria.

So what makes this the worst kind of “algae”?

Cyanobateria produce multiple different toxins which can effect the skin, liver and nervous system, and in rare circumstances – can cause death[2].

In addition to their toxin producing potential, BGA can form a blanket of slime that covers the surface of your pond. If this layer of bacteria grows enough, it can completely block out sunlight from the depths of the water.

Blue Green Algae (Cyanobacteria) covering a pond

This can completely disrupt the balance of an ecosystem and lead to a crash in oxygen levels and can result in fish-loss and increased mortality rates for all critters in your pond!

Fish, waterfowl, and other algae aren’t the only organisms affected by cyanobacteria.

Adults, children,  and pets exposed to BGA will have irritated skin and can suffer serious negative health effects.

*Not every BGA bloom is toxic, but this is one algae you do not want to mess around with so we recommend you learn how to prevent blue-green algae.

Not all Algae is bad!

Nualgi Natural Pond Cleaner starts a process using the power of diatoms (Mother Nature’s favorite microscopic cleaners) to recalibrate your pond chemistry & improve the health of the entire pond ecosystem. The growth of diatoms leads to more oxygen in the water which provides ponds with algae relief and a healthier habitat for fish. 

With more oxygen in the water, fish become healthy, vibrant, less disease-prone and more active. The result? A beautiful crystal clear pond that is a haven to fish and wildlife – and a joy to sit beside.


Filamentous Algae

  • Common Names: Carpet Algae / Hair Algae / String Algae / Spirogyra
    / Water Silk
  • Description: Long, thin, hair-like strands of algae that can combine to form mats
  • Where/how it grows: Anchors to the bottom of a pond or structures like rocks or waterfalls before floating to the surface in large mats
  • Region/Climate: Prefers fresh, clean water and cooler temperatures
  • Toxic: No
  • Predators: Some fish, including the Siamese Algae Eater, Pond Loach, and occasionally Koi

These algae can grow quickly and impact the appearance and ecosystem of your pond. While small amounts of filamentous algae are not harmful, it is important to control string algae before it grows out-of-control.

how to remove string algae from pond

Green Pond Algae

  • Common Names: Chlamydomonas, Chlorella, Closterium
  • Description: Free floating and microscopic, this type of algae blooms to give the water a green color, not to be confused with green water from new pond syndrome
  • Where/how it grows: In the water column, on the bottom, edges, and surface of ponds, visible during blooms
  • Region/Climate: Prefers warmer water
  • Toxic: No
  • Predators: Fish will not feed on this type of algae, but zooplankton and other microscopic organism will.
Planktonik Algae Being removed from Pond

Golden Algae

  • Description: Blooms will often give the water a golden color
  • Where/how it grows: Appears on the surface of the water
  • Region/Climate: Non-native, prefers temperatures between 65⁰F – 85⁰, occurs primarily in coastal waters
  • Toxic: Small amounts are not harmful; algal blooms are toxic to fish though not to animals or humans
  • Predators: Toxic to fish
Golden water from bloom

Red Pond Algae

  • Background: Like Cyanobacteria, red pond algae is technically not an algae, but a protozoan named Euglean that typically appears red in color.
  • Description: Euglena can be found in nearly all bodies of water and blooms are typically red in color, but may turn green
  • Region/Climate: Can grow in any body of water but prefers warm waters
  • Toxic: In large quantities
  • Predators: Toxic to fish
golf course Lake Prior to Nualgi Treatment - Aug 6, 2014

Some of the same types of algae that grow in ponds will also grow in a home aquarium, and just like pond algae, not all types of aquarium algae are created equal.

Algaes Mistaken for Plants

Plants and algae are very different, but there are some algae that are great at pretending to be what they’re not. Where algae are mostly unicellular, plants are multi-celled and have many different parts (stem, leaves, roots, etc).

While it is easy to mistake certain algae types for plants it does not mean that they are harmless. These algae lookalikes still need to be controlled to prevent damage to your pond.

Nitella / Stoneworts

  • Description: Branched algae that appear below the surface of the water; varying shades of green; soft to the touch
  • Region/Climate: Prefers warmer temperatures and acidic ponds
  • Purpose: Provides a habitat for insects eaten by fish in the pond; helps to stabilize sediment at the bottom of the pond
  • Beneficial or Detrimental: Beneficial in small amounts
Nitella Stonewort

Chara / Muskgrass

  • Description: Plantlike algae that appear below the surface of the water; gritty feeling and musky smell
  • Region/Climate: Prefers alkaline water
  • Purpose: Provides a habitat to insects eaten by fish in the pond; helps to stabilize sediment at the bottom of the pond
  • Beneficial or Detrimental: Beneficial in small amounts
Chara algae


These organisms are mistaken as algae, but are actually a small invertebrate which live in colonies which can resemble clumps of algae. These colonies are typically 2-4 inches long and attach to surfaces like rocks and branches. Each member of the colony has microscopic tentacles they use to filter feed.

They are found worldwide in every kind of water and are non-toxic. Freshwater bryozoans are preyed on by snails, insects, and fish and are good for your pond in limited quantities.


But What About Good Algae?

We thought you would never ask!

With an estimated 200,000 species known to exist, our favorite type of algae are Diatoms!

Diatom Algae

  • Description: These single cell organisms are one of the most common types of phytoplankton, and are found in just about every body of water – including freshwater to very salty waters — and just about every climate from the Arctic to the Equator!
  • Where/how it grows: Often free-floating on the surface; they later sink to the bottom of the pond and are consumed
  • Region/Climate: Most climates – from cold to warm – and most bodies of water – from fresh to briny
  • Purpose: Nutrition for zooplankton (and thus for fish); create oxygen
  • Predators: Zooplankton
diatoms algae control
diatom growth 15 years of research

What makes diatoms different?
Unlike most phytoplankton, diatoms have super special silica-based cellular structure which makes them different from all other types of algae you find in ponds.

We know what you’re thinking — “silica-based” — big deal.

Actually, it is! Unlike cellulose-based algae which form a layer on top of the water that cuts off light, diatoms silica body allows them to float in the water column and then sink to the bottom where they are eaten by your pond’s fishy inhabitants.

This good algae also converts nitrogen, phosphorus and carbon dioxide into oxygen-rich organic compounds that create a healthy ecosystem.

Did you know? Diatoms are responsible for producing up-to 50% of all oxygen on planet earth!

Watch the video from National Geographic below to learn more about the power of Diatoms!

We hope this article was helpful!

Here at Nualgi, we pride ourselves on being able to answer any questions you may have about keeping your pond healthy and free from bad algae.

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