9 Types of Pond Algae – Identification Guide

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Algae bloom is common in still waters like aquariums and ponds. It’s not a welcome occurrence, but aquarists have to deal with it nonetheless. If you’re continuously fighting to keep your pond clean, maybe it’s time to get to know your enemy a little bit better.

In this article, I’ll cover some of the most common pond algae species. Keep reading to learn about the nine most frequently occurring algae, how they look, and how they grow.  Most importantly, this information will help you decide whether your pond algae pose a threat to your fish.

Knowing all this will come in handy when deciding how to handle the situation. So, without further ado, let’s get started!

1. Green Algae (Chlorophyta spp.)

Unsurprisingly, green algae give the water a green color. There are over 7,000 species of green algae in the family Chlorophyta, but they’re all very similar appearance-wise.

These are micro-sized, free-floating plants. If you were to grab some out of the water, they’d have a mushy, shapeless consistency. If you get green algae bloom, the water will look muddied and opaque. The color of the water will range from dark to yellowish-green.

Ideal Parameters

Green algae are the most diverse and widespread group of water algae. They can grow in most still waters and climate conditions. They prefer warm water, and will quickly proliferate in temperatures 77- 86°F. This is why algae bloom happens during summer but slows down once the temperature drops.


Green algae are microscopic and free-floating. Their structure is one of the main reasons why they proliferate so fast. They spread throughout the water column and can form deposits on the edges, the bottom, and the pond’s water surface.

Pros and Cons

Green algae look inaesthetic and destroy water quality. Letting them unchecked can deplete the water of oxygen and subsequently kill other plants or fish in the pond.

But a few green algae might be beneficial. Green algae are a food source for a variety of small creatures (zooplankton, other microscopic organisms, and even some insects) living in or near the pond.

Fish Safety

Most fish won’t feed on this type of algae. However, green algae are non-toxic to fish. If your fish accidentally ingest some of it, there’s nothing to worry about. These algae can only pose a threat to fish if they affect the water quality.

2. String Algae (Filamentous Algae)

String algae, as the name suggests, are stringy and thin. This type of pond scum is made up of individual hair-like strands. These intertwine and stick together, forming spongy, loosely-knit mats.

Their appearance has inspired other fitting names, including hair algae, carpet algae, and water silk. String algae color ranges from bright to deep green.

Ideal Parameters

String algae prefer cooler temperatures and will grow in clean still waters. This type of algae often appears in spring and can start growing in temperatures as low as 36°F. Light exposure and water quality are most important for this type of algae. Most species thrive in soft water and under high light conditions.


These algae first start growing on the bottom of the pond. They spread onto rocks and other hard surfaces and continue to proliferate from there. Eventually, the growing mats start trapping oxygen bubbles and they float to the surface. Once dislodged, string algae remain free-floating and can easily take over the entire pond.

Pros and Cons

String algae spread like wildfire and can quickly deplete the water of oxygen. They also impact the appearance of the pond. Because of their texture, string algae can also clog water filters and other equipment.

The one good thing about these algae is they can make a good food source. Many pond fish such as Koi, Pond Loaches, and Siamese Algae Eaters enjoy nibbling on this aquatic plant. In the wild, these algae also feed waterfowl, zooplankton, and aquatic insects.

Fish Safety

String algae are non-toxic and highly nutritious. Many pond fish enjoy consuming them. They provide fiber, B-vitamins, and useful plant pigments that keep your fish healthy and vibrant.

Of course, you don’t want to let these aquatic plants colonize the entire pond. This can generate harmful ammonia and nitrates. The excessive algae growth can also deplete oxygen.

3. Blue-Green Algae (Cyanobacteria)

Blue-green algae are microscopic and buoyant, so they can easily colonize the surfaces of ponds and other still waters. They usually have a runny, mushy texture similar to green algae. But they can also grow stringy and form mats, similar to filamentous algae.

Despite their name, blue-green algae can be not only blue or green but also brown or reddish-purple. This type of algae bloom often emits an unpleasant odor that’s not characteristic of other algae species.

Ideal Parameters

An important detail to remember is that blue-green algae are not aquatic plants. This is a type of bacteria, more precisely cyanobacteria. Thus, BGA don’t need the same conditions as other algae.

They usually grow in shallow still water and prefer warm temperatures. They can grow in both fresh and salt water. As a bacterium, BGA thrives in unsanitary conditions. Excessive waste, low oxygen, and high nitrogen concentrations encourage BGA bloom.


Like all bacteria, BGA are microscopic. This means they need little energy to proliferate and they can spread anywhere through the water. If the conditions are right (high temperature, light exposure, and high nitrogen), this bacterium can multiply very quickly.

They can start growing in any spot of the pond. But they’ll usually be on the water surface or just below the surface due to their buoyancy.

Pros and Cons

Cyanobacteria is a natural part of all aquatic systems. It’s found in all soils and it serves an important role for aquatic plants. This bacterium helps fix nitrogen in the soil, making plant growth possible. Without this microscopic organism, plant life might not even exist!

Impressive as this sounds, cyanobacteria are also potentially dangerous if left to grow out of control. This bacterium is highly toxic and can even harm humans! The toxins produced by cyanobacteria cause skin irritation and can damage the liver and nervous system.

Fish Safety

A blue-green algae bloom requires immediate attention. This type of algae is extremely toxic and even deadly for fish. In fact, dead fish, foul smells, and dying plants are common signs of excessive cyanobacteria in the water.

BGA harm fish directly through the toxins they produce, but that’s not all! They can also take over and destroy the entire pond ecosystem. BGA, like other algae, can deplete oxygen levels, increase nitrate concentrations, and suck up important nutrients. They create unfavorable conditions that destroy all life in the pond.

4. Chara Algae (Muskgrass)

Chara algae are often mistaken for grasses and other aquatic plants due to their appearance. They look like thin, long-stemmed plants with many wispy offshoots, similar to hornwort.

These multi-cellular algae have a washed-out green color and can grow anywhere between one inch to 6.5’ tall. Because they aren’t true plants, Chara algae have no roots. But they can fix themselves on the bottom of the pond by creating dense, carpet-like tangled structures.

Ideal Parameters

This type of algae grows very tall, so it can thrive in both shallow and deep bodies of still water. It’s also well-adapted to both fresh and salt water. Chara algae only need two things to thrive— warm temperatures and alkaline water.

Like most algae, it will bloom throughout the summer months. Once temperatures drop, the growth slows down or the algae die. Because they like alkaline water, Chara algae aren’t a problem in soft water ponds.


These algae grow very similarly to plants. They aren’t buoyant, so they first start proliferating at the bottom of the pond. They quickly fix themselves onto the substrate or rocks, and their stem-like appendages start growing vertically towards the light.

Depending on the height of the pond, Chara algae can reach even 6.5’ feet in length. However, they never grow above the water surface.

Pros and Cons

Chara algae are prone to overgrowth, so they’ll quickly obscure the view in the pond. They also emit a strong, unpleasant garlic-like odor. I guess the nickname “Muskgrass” is fitting! But besides these disadvantages, Chara algae are quite beneficial.

They cover the bottom of the pond and help keep sediments in place. Thus, they prevent murky water. They improve water quality by filtering harmful compounds. They also produce extra oxygen for fish. Finally, they act as hiding spaces for fish and insects.

Fish Safety

Chara algae are non-toxic for fish. In the wild, these plant-like algae provide a habitat for insects, which are then consumed by fish in the pond. Fish occasionally bite into these algae by accident when hunting for food. But it won’t do them any harm.

5. Euglena Algae

Euglena are single-cell organisms. They are similar to green algae because they’re free-floating and congregate on the water surface to create a mushy film. But they have a distinct appearance. Euglena have a deep red pigment.

They give the water an unmistakable crimson color. Euglena algae can also sometimes appear green. Under high light exposure, the algae will be bright red. The color can fade to green if the light exposure drops.

Ideal Parameters

Euglena are highly invasive and can grow in virtually any type of water and climate. They can even grow in moist soils. But they prefer fresh and still water. The water parameters (hardness, pH, temperature) don’t matter. But warm water does encourage large blooms.


Much like blue-green algae, euglena can be insidious. They’re free-floating, so they can start growing anywhere. Moreover, they can potentially survive and thrive in any type of water.

The most concerning thing is that you won’t know it’s in the water until it starts to bloom. This red algae bloom can appear all of a sudden and will typically be visible on the water surface. Once the algae establish themselves, they will proliferate rapidly.

Pros and Cons

There’s no known pro of this algae. If this scum grows in your pond, it’s a sign that something is wrong with the water quality. Euglena are resilient, can only be removed through aggressive means, and contribute nothing to the pond’s ecosystem.

Fish Safety

Besides being utterly useless, euglena bloom is also highly toxic to fish. Exposure to these algae can kill fish, aquatic plants, and virtually any lifeform in the pond. To make matters worse, you’re more or less required to kill everything in the pond to get rid of this intruder.

You can’t remove them manually. You’ll have to drain all the water and disinfect the pond with sodium carbonate or copper-containing solutions. This will take care of the algae, but will also kill all the beneficial bacteria and flora.

6. Golden Algae

Golden algae are microscopic, single-celled algae with, you guessed it, a golden color! They give the water a yellow appearance thanks to their high concentration of carotenoid pigments.

Unlike other free-floating algae on the list, golden algae rarely form a scum on the water. Golden algae give the water a muddy appearance similar to dislodged, free-floating sand.

Ideal Parameters

Golden algae occur primarily in coastal fresh waters and thrive in warm temperatures (65-85 °F). Despite their common occurrence in fresh water, researchers have observed that high salinity, sulfate, and chloride concentrations encourage golden algae bloom.

These algae can also be found in rivers and lakes. Very rarely, they might be found in ponds as a result of accidental contamination. It’s non-native to the U.S. It’s first been documented in Pecos River, Texas in 1985 and has since spread to other bodies of water.


Golden algae are free-floating. They appear and proliferate close to the water surface. When the conditions are optimal, they can bloom in large numbers. These algae rarely create scum on the water. But the telltale signs of bloom are a yellow tint and decreased water transparency.

Pros and Cons

There are no known pros of golden algae. At best, this type of algae is benign when present in small quantities. Most golden algae blooms rarely cause water oxygen depletion. Compared to other algae, they’re less harmful to other plants in the pond.

Golden algae are very difficult to get rid of. You can’t remove or control them mechanically because they’re too small. You must clean the entire pond and replace the water. By far the biggest con of golden algae is their toxicity.

Fish Safety

Golden algae pose no threat to humans and most wildlife. But they’re highly toxic to fish. When exposed to these algae, fish will present bleeding gills and scales.

Golden algae don’t cause oxygen depletion, but still induce suffocation in fish. And since they’re microscopic and free-floating, the fish will inevitably swallow them.

7. Bryozoans

Bryozoans have a peculiar appearance compared to other algae. And that’s because they aren’t real algae, but small invertebrates. Nevertheless, they have many traits that would make you confuse them with algae. That’s why they’re often categorized as such.

Bryozoans live in colonies and will appear like smooth, slimy clumps attached to branches, rocks, and other objects in the pond. These colonies are usually 2-4 inches long and come in a variety of colors including white, dirty yellow, grey, or light brown.

Ideal Parameters

These algae-like invertebrates can be found worldwide in any type of water. They mostly prefer still waters like lakes and ponds. Colonies grow best in clean, warm, and soft water. They’re also most common in waters with a neutral or alkaline pH.

But bryozoans can survive in a wide variety of water parameters, including acidic, hard, cold, or high-salinity water. Certain species also thrive in murky waters. Since they aren’t real algae, the light exposure in the pond won’t have a big impact on their growth.


Bryozoans are microscopic animals, so you can expect them to behave as such. They mostly reproduce sexually. Their breeding season is in late summer or early fall. When temperatures are at their peak, bryozoans produce eggs.

As the temperatures begin to fall, the colony starts dying off. The eggs can survive through the winter. The newly-hatching bryozoans will form new colonies in spring and summer. Once hatched, the new bryozoans can also reproduce asexually through budding.

Pros and Cons

These “algae” don’t look aesthetically pleasing. But there’s nothing dangerous about them. If anything, they’re a sign of good water quality in the pond. The best thing about them is that they filter-feed. They eat bacteria and zooplankton, thereby preventing waste by-products like ammonia and nitrites.

Bryozoans have a cascade effect that keeps the entire pond ecosystem healthy. Their filtering power maintains good water quality and keeps algae at bay. Bryozoans themselves are also a safe source of nutrition for insects, snails, and fish.

Fish Safety

If you see a colony growing in your pond, there’s no reason to panic! Bryozoans are non-toxic to fish. Many fish species consume these organisms without issue. Bryozoans are also eaten by snails and insects.

8. Nitella Algae (Stoneworts)

Nitella is another species of plant-like algae. These algae look similar to Muskgrass (Chara algae), but there are a few distinctions. Nitella algae are light to dark green and can reach between 3’’ and 6’ in height.

They have thin, flexible, stem-like structures and forked bushy branches less than 1/8 inches in diameter. Like Chara algae, Nitella won’t extend above the water’s surface. But unlike their musky counterpart, Nitella algae are odorless.

Ideal Parameters

Nitella algae grow in still waters such as lakes and ponds. They prefer fresh and warm water, so you’ll rarely see them in high salinity conditions. A special thing about them is that they thrive in slightly acidic or acidic ponds.

Given their need for acidic water, Nitella algae are more common in poor water conditions. A dirty pond with poor filtration is more likely to have acidic water. Bacteria and waste can create acid, which pulls the pond’s pH down.


Nitella algae have small, root-like structures at the base of their “stems”. They use these fake roots to anchor themselves into the substrate or on rocks, and other surfaces in the pond.

Like most other algae species, Nitella can grow rapidly once it’s established itself in the pond. Most people mistake them for regular plants and let the algae go unchecked until the bloom is out of control.

Good light exposure, suitable water parameters, and high nutrient concentrations in the water encourage excessive growth. What is more, this species can propagate through fragmentation, but also sexually. All it takes is one algae segment to create an entire mini-lawn in your pond!

Pros and Cons

This algae species is completely benign, besides the annoying, rapid growth. If left unchecked, Nitella can obscure the water and make your maintenance job more difficult. But they don’t pose a threat to water quality or animals in the pond.

Nitella provides a habitat for insects and small aquatic critters. It can also serve as a hiding spot for the fish in the pond. The root-like structures keep sediment in place, thereby preventing murky water.

Fish Safety

Nitella is non-toxic to fish and wildlife. Some fish species enjoy snacking on the plant directly. Even fish that don’t consume a lot of algae will benefit from it. Nitella can house several insects and other invertebrates. And most fish consume these!

9. Diatom Algae

Diatom algae are invisible to the naked eye. These are microscopic, single-cell organisms that leave no traces. They’re the smallest of all types of algae. You might have also heard them described as “phytoplankton” or “microalgae”.

You can only see them under a microscope. However, when the diatom algae concentration in the water is high, you might notice faint, clear patches of green on the water. That’s the chlorophyll in the algae reflecting the light.

Ideal Parameters

According to fossil records, diatoms might have been around since the early Jurassic period. These microscopic algae are 200 million years old. It’s no surprise that they’re adapted to withstand just about anything.

Diatoms grow in any body of water and any climate imaginable. You’ll see them (or not) in fresh, briny, alkaline, acidic, warm, or very cold water. In nature, they’re present in lakes, wetlands, streams, rivers, oceans, and estuaries. Unsurprisingly, they also occur in artificial ponds.


Diatom algae are very small, but they work like any other type of algae. Their growth depends on available nutrients in the water. Carbon dioxide, nitrogen, and sunlight are particularly important. With the right nutrition, phytoplankton can reproduce extremely quickly. These microalgae can double in number in just one day.

They reproduce asexually through mitosis (cell division). The reproduction and feeding process often starts at the surface. New diatoms are free-floating and soaking up sunlight at the water surface. Later, they sink to the bottom of the pond where they become food for other microorganisms and small invertebrates.

Pros and Cons

Diatom algae are a crucial part of aquatic ecosystems. They fulfill many useful functions such as water filtration and generating oxygen. They also consume acidifying bacteria, thereby contributing to stable water pH.

Finally, phytoplankton also play an important role in the food chain. These microalgae are eaten by zooplankton. Later, zooplankton also serve as food for fish and other critters in the pond. The only downside is that diatoms in high concentrations might turn the water an unsightly green color.

Fish Safety

Diatom algae are completely safe and also beneficial for fish. They indirectly serve as food. They also maintain good water quality and keep bacteria in check. But if you want to get rid of them, you can install a UV water sterilizer.


There are many types of algae you can encounter in a pond. Some of them are algae proper, while others are commonly classified as such for convenience. Blue-green algae and bryozoans, for example, aren’t algae at all. They’re bacteria and plankton, respectively. But they impact the water quality similarly to real algae.

Most common algae blooms will stain the water in different colors, ranging from green to yellow and even red. These colorful algae are the most dangerous because they’re toxic and lethal to fish. Blue-green algae, golden algae, and red algae (euglena) all require emergency intervention.

Other algae are non-toxic, but can still deplete water oxygen and harm your fish. This category includes string algae and green algae. Finally, certain algae can be beneficial for the pond’s ecosystem if present in small quantities. This includes Chara algae, Nitella algae, bryozoans, and diatoms.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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