What Actually Works Against String Algae? I Tested 8 Methods to Find Out

String algae can be one stubborn bugger in a backyard pond. So I decided to run a little home experiment to test out different algae control methods — all using water from my own courtyard pond.

And yes, I raided Mrs OzPond’s pantry for jars…

But before we dive into the results, a quick heads-up: if you’re new here, I’m Kev, and I help people build and maintain ponds without spending a fortune.

If you want to learn how to do that you’ll love my free newsletter.

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🧪 The Algae Experiment Setup

I filled 9 jars with pond water from a well-established 6-year-old pond and added the same amount of stubborn string/hair algae to each. Then I tested these 8 methods:

1. Control (no treatment)
2. Shade
3. Plants (Hornwort + Red Root Floater)
4. Substrate from a mature pond
5. Shrimp
6. Liquid Bacteria
7. Diatoms (via a product called diatomix)
8. Sodium Percarbonate
9. Copper-Based Algaecide

Each jar sat on a kitchen windowsill with good light but no direct sun, except one that I moved to deep shade.

You can find links on where to buy these products by clicking this link. Then, scroll down to the water treatments section.

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🌱 What I Expected

• Some of these I was skeptical about (like shade and liquid bacteria).
• Others — like plants and sodium percarbonate — I had higher hopes for.

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🔍 Day-by-Day Observations

Day 1

• Copper algaecide jar already looked bleached.
• Sodium percarbonate jar was full of oxygen bubbles — but the algae wasn’t dying yet.
• Bacteria and shrimp jars showed signs of activity.
• All others: no change.

Day 3

• Plant jar: Algae was basically disintegrating. Hornwort seems to live up to the hype.
• Bacteria jar: Algae shrinking.
• Sodium percarbonate: Starting to break down algae.
• Algaecide: Algae was brown and dead-looking.
• Others: not much change.

Day 6

• Sodium percarbonate: Water clear. Algae gone.
• Plants: No string algae left — just fine powder.
• Algaecide: Algae dead but still intact (no breakdown happening).
• Control: Starting to grow new algae.
• Substrate, shrimp, diatoms: Maybe slight improvement, but very slow.

If you would like to watch a video on this experiment, here it is:

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✅ What Worked Best?

Here’s how they stacked up:

Method	Effectiveness	Notes
Sodium Percarbonate	⭐⭐⭐⭐⭐	Completely cleared algae. Oxidizer. Quick action.
Plants (Hornwort)	⭐⭐⭐⭐⭐	Very effective, natural, and safe. Cleared the jar by day 6.
Bacteria	⭐⭐⭐⭐⭐	Slower start, but after two weeks it cleared the algae almost completely. Excellent long-term solution.
Algaecide (Copper)	⭐⭐⭐⭐☆	Effective but toxic to pond life — snails, shrimp, and other critters.
Diatoms	⭐⭐☆☆☆	Some slight improvement, but nothing dramatic.
Shrimp	⭐⭐☆☆☆	Possibly helped, but couldn’t keep up.
Substrate	⭐☆☆☆☆	Small effect over time. Needs more conditions to thrive.
Shade	⭐☆☆☆☆	Prevented new growth but didn’t reduce existing algae.
Control	☆☆☆☆☆	Business as usual — algae was still partying.

If you would like to try sodium percarbonate in your pond you should watch this video first:

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🧪 Update on Bacteria Treatment

While it didn’t show dramatic changes during the first few days, by the two-week mark, the jar treated with liquid bacteria had cleared up significantly.

The algae had broken down and the water was much clearer — showing that bacteria-based treatments may take longer to work, but can be very effective over time.

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🧠 Final Thoughts

Sodium percarbonate was — fast, effective, and it literally vaporised the algae.

Plants like hornwort also did an amazing job, plus they’re safe for all pond life.

Copper-based algaecide worked too, but it’s not something I will use in my ponds due to its impact on snails, shrimp, and other small creatures.

That said, the long game in any pond is balance — substrate, plants, animals, and good biological filtration (like a bog filter).

Over time, this setup creates a self-sustaining ecosystem where algae struggles to take over.

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💡 Want a Pond That Looks After Itself?

Grab my Pond Formulas Blueprint — it’s the same guide I use to design natural filtration systems for crystal-clear, low-maintenance ponds.