Pond algae generally falls into four categories: planktonic algae (cloudy green water), filamentous algae (stringy mats, also called string algae), Chara (a crunchy, rooted-looking macroalgae), and blue-green algae or cyanobacteria (a paint-like film that can be toxic). Identifying which one you have matters because a treatment built for one type often won't work on another, which is why so many pond owners feel like they've "tried everything" without success.
What Is Planktonic Algae?
Planktonic algae is made of microscopic, free-floating cells suspended throughout the water column, and it's what causes the classic "pea soup" look, cloudy green water ranging from bright to olive. It's the most common form of algae and, in small amounts, is actually a normal part of the pond food chain. It becomes a problem when it blooms heavily enough to cloud the entire pond and strip oxygen as it decomposes.
What Is Filamentous (String) Algae?
Filamentous algae, commonly called string algae, forms long thread-like strands that start along the pond bottom and intertwine into mats resembling wet wool or hair. When these mats float to the surface, they're often called pond scum. Unlike planktonic algae, string algae doesn't cloud the whole pond, it clings to rocks, liner, and waterfalls in visible clumps.
What Is Chara (Muskgrass)?
Chara is a macroalgae that's frequently mistaken for a rooted aquatic weed because of its plant-like, branching structure. The giveaway is texture and smell: Chara feels gritty or crunchy due to calcium deposits on its structure, and it often has a distinct musky or garlic-like odor when crushed. Because it looks like a plant but is actually algae, a standard aquatic herbicide built for rooted weeds often isn't the right tool, Chara typically responds better to a copper-based algaecide.
What Is Brown Algae (Diatoms)?
Diatoms, often called brown algae, form a dusty, golden-brown to tan film on rocks, pond walls, and slower-growing plants, and they usually wipe off easily with a finger, unlike the gritty, harder-to-remove texture of Chara. Diatoms are generally considered harmless and are common in new ponds or in early spring as a pond's ecosystem is getting established. They're often left alone rather than treated, since they typically fade on their own as the pond matures or as other algae types become more dominant with warmer weather.
What Causes Reddish or Ruddy Pond Water?
A reddish or ruddy tint is most often caused by Euglena, a type of algae common in nutrient-saturated ponds. Euglena can migrate up and down in the water column throughout the day to avoid intense afternoon sun, so a pond can visibly shift color at different times of day. True red algae is genuinely rare in freshwater backyard ponds, and a reddish tint is sometimes mistaken for it when the actual cause is tannins from decaying leaves and organic debris (which give water a tea-colored tint) or, less commonly, a reddish strain of cyanobacteria. Since cyanobacteria can't always be ruled out by color alone, treat a persistent reddish bloom with the same caution as blue-green algae until you've identified the actual cause.
What Is Blue-Green Algae (Cyanobacteria), and Is It Dangerous?
Blue-green algae, technically a bacteria rather than a true algae, can appear as a turquoise, lime-green, or paint-like film on the water's surface, sometimes with a strong, musty odor. Some varieties of blue-green algae produce toxins that can be harmful to pets, livestock, and people, so it's treated differently than the other three types. If you suspect blue-green algae, keep people and pets away from the water and out of contact with it. Aggressive treatment isn't recommended, since a large amount of algae dying off at once releases toxins as it decomposes and can also trigger the same oxygen crash that leads to a fish kill. In most cases, a blue-green algae bloom resolves on its own as water cools, and identification or treatment guidance from a water quality professional is the safer route if you're unsure.
This Looks Green, But It's Not Algae
Not everything that turns a pond green is algae. Duckweed and watermeal are two of the most common impostors, both are floating plants, not algae, even though a heavy covering of either can look like a solid green film across the water's surface from a distance. The tell is up close: duckweed has tiny, individual round or oval leaves, often with a single thin root hanging below each one, while watermeal is even smaller, appearing as fine green grains without visible leaves or roots at all. Algae, by contrast, forms a uniform cloud or film with no distinct individual plant structures.
This distinction matters because the products that treat algae generally won't touch a floating plant like duckweed, and vice versa. For the full breakdown on telling algae apart from pond weeds, including duckweed, and which product fits each, see Is It Algae or a Pond Weed? How to Tell the Difference.
Quick Reference: Look, Feel, and Smell
| Algae Type | Look | Feel | Smell |
|---|---|---|---|
| Planktonic | Cloudy pea-soup green water | N/A, suspended in water | Mild, earthy |
| Filamentous (string) | Stringy mats on rocks, liner, waterfalls | Slimy, hair-like | Mild |
| Chara | Branching, plant-like growth | Gritty or crunchy | Musky or garlic-like |
| Diatoms (brown) | Dusty golden-brown to tan film on rocks and surfaces | Wipes off easily | Mild to none |
| Ruddy/red (often Euglena) | Reddish or rust-tinted water, may shift through the day | N/A, suspended in water | Mild |
| Blue-green (cyanobacteria) | Turquoise, lime-green, or paint-like film | Not recommended to touch | Strong, musty |
| Duckweed (not algae) | Tiny individual round leaves covering surface | Distinct small leaves, not a film | Mild |
How Do You Get Rid of Each Type?
- Planktonic algae: a fish-safe algaecide clears cloudy green water, usually within 24 to 48 hours, followed by beneficial bacteria and aeration to keep it from returning.
- Filamentous (string) algae: the same algaecide products work on string algae, and manually removing loose mats once they detach speeds up results.
- Chara: a copper-based algaecide is typically more effective than a standard aquatic herbicide, since Chara is algae rather than a rooted plant, despite its plant-like appearance.
- Diatoms (brown algae): usually don't require treatment, since they're harmless and tend to fade as a pond matures or warms into the next algae season. Manual wiping or a light scrub is enough if they're bothersome on visible surfaces.
- Ruddy or red water (Euglena): treat the same as planktonic algae with a fish-safe algaecide, since Euglena responds to the same products. If the reddish tint persists after treatment or you're unsure whether it's Euglena versus cyanobacteria, treat it with the same caution as blue-green algae until confirmed.
- Blue-green algae: avoid aggressive DIY treatment. Keep people and pets away from the water, and reach out to our team or a local water quality professional for guidance specific to your situation before treating.
Whichever type you're treating, avoid treating a heavily bloomed pond all at once. Decomposing algae pulls oxygen out of the water as it breaks down, and treating in sections with aeration running keeps oxygen levels stable while the pond clears. For a full walkthrough of fish-safe treatment timing, see How to Get Rid of Green Pond Water Without Killing Your Fish.
Products for Algae Treatment
- Cutrine Plus Liquid: a copper-based algaecide effective on planktonic algae, string algae, and Chara.
- Captain XTR: fast-acting algaecide labeled for use in ponds with fish.
- Green Clean Granular: a contact algaecide useful for spot-treating string algae mats.
- Pond Dye: blocks the sunlight algae needs to grow, a useful long-term prevention step once water is clear. Worth noting: some pond management professionals prefer to skip dye specifically because it masks the pond's natural water color, which is one of the easiest ways to visually monitor algae activity over time, so it's a trade-off worth considering rather than an automatic add.
- Pond Aeration Systems: supports oxygen levels during treatment and helps prevent regrowth across all algae types.
Not Sure What You're Looking At?
Send us a photo or describe what you're seeing (color, texture, smell) by clicking the Ask Our Pond Pro button in the bottom left corner of any page on thepondshop.com. We'll help you identify it and point you to the right product.
Call: (800) 527-9420 Email: sales@thepondshop.com
Related Reading from The Pond Shop
- Is It Algae or a Pond Weed? How to Tell the Difference
- Why Pond Muck Causes Algae (and How to Get Rid of It)
- Why Is My Pond Water Green?
- How to Get Rid of Green Pond Water Without Killing Your Fish
FAQ Section
What are the main types of pond algae? Pond algae generally falls into four categories: planktonic algae (cloudy green water), filamentous or string algae (stringy mats), Chara (a crunchy, rooted-looking macroalgae), and blue-green algae or cyanobacteria (a paint-like film that can be toxic).
How do I know if I have string algae or green water? String algae forms visible stringy mats that cling to rocks, liner, and waterfalls. Green water (planktonic algae) clouds the entire pond rather than clumping in one spot.
Is Chara a plant or an algae? Chara is a macroalgae, not a rooted aquatic plant, even though its branching structure makes it look plant-like. It typically feels gritty or crunchy and has a musky or garlic-like smell, and it usually responds better to a copper-based algaecide than a standard aquatic herbicide.
Is blue-green algae dangerous? Some varieties of blue-green algae (cyanobacteria) produce toxins that can be harmful to pets, livestock, and people. Keep people and pets away from water you suspect contains blue-green algae, and avoid aggressive treatment, since dying algae can release toxins and deplete oxygen at the same time.
Is brown algae in my pond dangerous? No. Brown algae, called diatoms, is generally harmless and common in new ponds or early spring. It typically wipes off easily and often fades on its own as the pond matures.
Why is my pond water reddish or rust-colored? Reddish or ruddy water is most often caused by Euglena, a type of algae common in nutrient-rich ponds. It can also come from tannins released by decaying leaves. True red algae is rare in freshwater ponds, and a persistent reddish tint that doesn't clear should be treated with the same caution as blue-green algae until the cause is confirmed.
Why isn't my algaecide working on the algae in my pond? Different algae types respond to different treatments. A product effective on planktonic or string algae may not work well on Chara, and standard algaecides aren't the recommended approach for blue-green algae. Correct identification is the first step to effective treatment.
