How to Get Rid of String Algae (Blanketweed)

String algae, also called blanketweed, grows in long green strands clinging to rocks, waterfalls, and plants, and unlike green water it will not respond to a UV clarifier because it never flows through the unit. The real fix is a combination: pull it out by hand, starve it by cutting nutrients and adding competing plants, dose beneficial bacteria, and use barley straw as a preventative. No single product clears it, but a consistent, balanced approach keeps it in check.

Your String Algae Toolkit

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CrystalClear Algae D-Solv Algae Control

$54.99 on Amazon

Fast-acting, copper-free algaecide effective on string algae and blanketweed; koi-safe when used as directed.

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API POND ALGAEFIX (1 gallon)

$41.29 on Amazon

Copper-free algaecide that controls string and hair algae across larger ponds; aerate during use.

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Aquascape Beneficial Bacteria Concentrate

$31.99 on Amazon

Competes with algae for nutrients and digests the sludge that feeds blanketweed.

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Pond Worx Pond Bacteria for Large Ponds (Gallon)

$26.99 on Amazon

High-volume beneficial bacteria for big koi ponds and water features; koi-safe.

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What string algae is, and why it is so stubborn

String algae is filamentous algae. Instead of floating as single cells the way green-water algae does, it grows into long, hair-like green threads that anchor to any surface they can grab, rocks, gravel, waterfall faces, plant baskets, pump housings, and netting. The strands knit together into slimy mats that can grow startlingly fast in warm, sunny, nutrient-rich water. The water around them often stays perfectly clear, which is exactly why string algae frustrates so many keepers: the pond looks clean except for the green beards on every rock.

Rock-and-gravel ponds are especially prone because all that rough surface area gives the filaments endless places to attach. The good news is that string algae responds to a patient, multi-pronged plan. The bad news is there is no magic bottle, and one tool in particular does not help at all.

Why UV does not kill string algae

This is the single most important thing to understand. A UV clarifier kills only the algae that physically flows through its chamber, where the ultraviolet light can reach the cells. That describes the free-floating single-celled algae behind green pond water, which is why UV clears green water so reliably. String algae is the opposite: it grows attached to surfaces and never enters the UV unit, so the light never touches it. Buying a UV clarifier to fight blanketweed is a common and expensive mistake. Save the UV for green water and reach for the methods below instead.

The string algae plan that actually works

1. Remove it manually

Physical removal is the fastest way to cut the mass right now. The filaments twist neatly around a stick or a stiff brush, almost like spaghetti on a fork, so plunge in and twist it out. Net the loose strands off the surface, and brush it off rocks and the waterfall. Removing the bulk also removes the nutrients locked inside it, so do this regularly during a bloom rather than once.

2. Starve it of nutrients

String algae feeds on nitrate and phosphate from fish waste, food, and decaying debris. Lower the supply and the regrowth slows:

• Feed less. Overfeeding is the biggest nutrient source. Feed only what koi finish in a couple of minutes, and ease off in cold water.
• Do not overstock. Koi are heavy-waste fish. Crowding the pond floods it with the nutrients algae craves.
• Remove debris and sludge. Net leaves, vacuum the bottom, and clean the skimmer. Rotting organics release nutrients for months.
• Do partial water changes. Weekly changes of 10 to 20 percent export nitrate. Always dechlorinate the new water.

3. Add competition with plants

Plants and string algae fight over the exact same nutrients. A well-planted pond, with floating plants, marginals, and especially a bog or planted filter, simply outcompetes the algae for food. Heavy planting that also shades the surface is one of the most effective long-term controls there is.

4. Dose beneficial bacteria

A strong beneficial bacteria colony competes with algae for nutrients and digests the organic sludge that releases them, lowering the food supply over time. Bacteria will not strip existing mats off your rocks, so this is a background measure that you pair with manual removal, not a standalone cure. For bigger ponds, a high-volume bacteria product covers the water more economically.

5. Use barley straw as a preventative

Barley straw is a slow, gentle inhibitor. As it decomposes it releases compounds that appear to discourage new algae growth, and it works best against string algae when added early in the season, before the bloom takes hold. It takes several weeks to act and will not remove existing strands, so treat it as ongoing maintenance, not an emergency treatment.

6. Knock back heavy growth with a koi-safe algaecide

For a severe infestation, a copper-free, EPA-registered algaecide can knock the mats back quickly while you work on the underlying balance. Two rules are non-negotiable:

• Dose to your real volume. Confirm your gallons with the pond volume calculator and follow the label exactly.
• Aerate hard during and after. Dying algae consumes oxygen as it breaks down, which can suffocate koi in a warm, stocked pond. Keep the waterfall and air pump running.

Keeping it under control for good

String algae is rarely eliminated entirely, and a little growth in a balanced pond is normal and harmless. The goal is control: enough plants and bacteria competing for nutrients, modest feeding, conservative stocking, clean debris, and occasional manual removal so it never builds into oxygen-robbing, pump-clogging mats. Keepers who win against blanketweed do a little consistently rather than waging one big chemical war.

String algae is a marathon, not a sprint. Pull it out, starve it, plant against it, and let beneficial bacteria and barley do the slow work, but do not waste money on UV, which cannot touch it. For the floating-algae cousin, see how to fix green pond water, and for the full strategy across both types read how to control pond algae.