How to Get Rid of String Algae
String algae (blanketweed) clings to rocks and waterfalls and a UV clarifier will not touch it. Beat it with manual removal, barley straw, beneficial bacteria, and better balance.
Fast answer: String algae is caused by excess nutrients plus sunlight, and unlike green water a UV clarifier will not kill it because the strands never pass through the unit. Your first action: physically pull out as much as you can by twisting it onto a brush, then attack the root cause with barley straw, beneficial bacteria, more plants, and tighter nutrient control.
String algae, also called blanketweed or hair algae, is the long green slime that wraps around your rocks, clings to the waterfall, and forms floating mats on the surface. It is different from the pea-soup look of green water, and it needs a completely different approach. Reaching for a UV clarifier here is the most common wasted purchase in the hobby. Here is what actually works.
What Helps Against String Algae
$11.31 on Amazon
Copper-free treatment that helps control string, hair, and blanketweed algae; aerate while dosing.
Summit Clear-Water Barley Straw, 2-Pack
$9.59 on Amazon
Slow-release barley straw that suppresses new algae growth as it breaks down.
Summit Clear-Water Barley Straw Bale (5,000 gal)
$20.99 on Amazon
Larger barley bale for bigger koi ponds; place near moving water in spring.
EasyPro Pond Products Barley Straw Pellets (Fast-Acting)
$77.98 on Amazon
Concentrated barley pellets for faster, easier dosing than loose straw.
Why string algae will not quit
String algae is an opportunist. It anchors to a surface, grabs whatever nutrients drift past, and uses sunlight to grow fast, especially in the warm, oxygen-rich, fast-moving water of a waterfall or stream. That is why you often see the worst growth exactly where the water tumbles. A few facts to set expectations:
- A UV clarifier does nothing to it. UV only works on suspended algae that flows through the unit. Anchored strands never get there.
- You will not eliminate it forever. The realistic goal is control and balance, not zero. Even pristine ponds get some.
- Clear water can still grow string algae. It thrives on the trace nutrients that leave the water column looking clean.
How to diagnose it
Look at where and how the algae grows. String algae forms long green threads or thick cottony mats attached to rocks, the liner, plant pots, and the waterfall. Pick some up: it feels stringy or slimy and comes out in clumps. If instead your water is uniformly green and you cannot grab any strands, that is green water, and you should read the green water fix instead.
Test your water too. High phosphate and nitrate are the fuel for string algae, often from overfeeding, overstocking, or decaying leaves and sludge. Knowing your numbers tells you how hard you need to push on nutrient control.
The string algae action plan
1. Remove it manually first
This is the fastest visible win and it matters before any chemical treatment. Twist the strands onto a stiff brush or notched stick, winding them up like spaghetti, and pull mats off rocks and the waterfall by hand. Removing the bulk first means less algae will die and decompose at once, which protects your oxygen levels and keeps the water from fouling.
2. Add beneficial bacteria
Beneficial bacteria compete directly with algae by consuming the same dissolved nutrients and breaking down the sludge that feeds it. Regular dosing through the warm months starves string algae from below. It is one of the most underrated tools for long-term control and it is completely safe for koi.
3. Use barley straw for ongoing suppression
Barley straw, loose or in pellet form, slowly releases algae-suppressing compounds as it decomposes. It works on prevention, not instant kill, so add it in early spring before the season's growth begins and place it near moving water where oxygen helps it break down. Replace it every few months and dose for your real gallons.
4. Spot-treat with an algae control product
For stubborn mats, a copper-free string algae treatment can help. Dose to your actual pond volume, never estimate, and run extra aeration throughout. Decomposing algae pulls oxygen out of the water, so strong aeration during and after treatment is essential to keep your koi safe.
5. Fix the balance for good
Lasting control comes from changing the conditions string algae loves. Add more plants to outcompete it, reduce feeding, remove sludge and leaves, and keep your filter and skimmer maintained. Some keepers also use pond salt at a low maintenance level for general fish health, which you can dose precisely with the Pond Salt Calculator.
How to prevent it from returning
- Plant heavily. Strong plant coverage steals the nutrients string algae needs.
- Feed and stock conservatively. Less waste means less fuel. Confirm your numbers with the Koi Stocking Calculator.
- Keep beneficial bacteria going. Dose regularly through spring and summer.
- Clean out debris. Skim leaves, vacuum sludge, and rinse filter media on schedule.
- Add barley straw each spring. Get ahead of the growth before it starts.
- Keep aeration strong. Healthy oxygen and circulation support the bacteria that compete with algae.
String algae and green water often get confused, so if your real issue is cloudy or pea-soup water, see how to fix green pond water and the deeper water-quality guide. Keeping fish indoors? Our sister site FishTankCalculator.com covers aquarium algae control.
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Frequently Asked Questions
Will a UV clarifier kill string algae?
No. A UV clarifier only treats free-floating single-cell algae that flows through the canister, which is what causes green water. String algae anchors itself to rocks, liner, and the waterfall, so it never passes the bulb. Running UV on a string-algae pond does nothing for the strands. Use manual removal, barley straw, beneficial bacteria, and better nutrient balance instead.
How do I remove string algae by hand?
Twist it out. Push a stiff brush, a toilet brush, or a notched stick into the mat and turn, winding the strands around it like spaghetti on a fork. Pull from waterfalls and rocks where it grips hardest. Remove as much as you can before any treatment, since killing a large mass at once can crash oxygen levels and foul the water.
Does barley straw really work on string algae?
Barley straw helps prevent new growth rather than killing existing strands. As it slowly decomposes it releases compounds that suppress algae, so it works gradually over weeks, not overnight. Add it in early spring before algae takes off, replace it every few months, and use enough for your gallons. Treat it as part of a balance strategy, not a quick fix.
Why do I get string algae but clear water?
That is actually common and often a sign of a healthy, well-filtered pond. Clear water means few suspended nutrients, but string algae is very good at grabbing the small amount of nutrients that remain, especially in sunny, fast-flowing spots like waterfalls. Beneficial bacteria, phosphate control, more plants, and manual removal bring it back into balance.
Will adding more plants reduce string algae?
Yes, over time. Submerged oxygenators, floating plants, and marginals all compete with string algae for nitrate and phosphate, and floating plants shade the water to limit light. A pond with strong plant coverage and a mature biological filter gives string algae far less to feed on, so the strands grow back slower and thinner after each removal.
Is string algae harmful to koi?
In small amounts it is harmless and even provides some grazing and cover. The risk comes from large mats: as they die and decompose they consume oxygen and can trap fish or clog pumps and skimmers. Keep it managed, never let a thick blanket take over the surface, and make sure aeration stays strong, especially in summer heat.
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