Understanding green water, brown water, string algae, and cloudy water — and how to fix each one.

If you want to understand why your pond isn’t clear, the first step is figuring out what you’re dealing with. Is the problem algae? Too much organic material? Or is there simply too much fine sediment floating around?

Every cause has a different solution — and in some cases, you might not need to do anything at all.

But if you don’t already know me, my name is Kev. My aim is to help people build and maintain their ponds without spending a fortune. If that sounds like something that interests you, you might like to subscribe to my YouTube channel and visit ozponds.com.

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Green Water (Pea Soup Algae)

Green water almost always means the filter is either undersized or overwhelmed. And just to be clear — green watermeans the water itself is green when you scoop it up. This is different from string algae, which is green and slimy but usually grows while the water stays clear. We’ll get to that later.

Green water is caused by tiny single-celled algae feeding on nutrients and sunlight. To stop it, you only need to remove one of those two ingredients.

Why I don’t recommend “quick kills”

You can kill green water directly — UV clarifiers, chemicals, shading — but I prefer not to. Green water is telling you something important:
Your pond has an imbalance.

And green water isn’t harmful. In fact, it’s providing heaps of natural food for tiny organisms like daphnia and small fish. They feed bigger fish, and bigger fish feed birds and wildlife. It’s nature’s way of recycling excess nutrient.

But when the nutrient level keeps increasing, the system eventually collapses — the same way wetlands can build up too much sediment and eventually turn into meadows unless floods flush them out.

Our ponds are little versions of that same process. Too much nutrient = too much growth = instability.

How I manage nutrient naturally

I prefer designing ponds that let natural processes do the heavy lifting:

• Understock the pond
• Don’t overfeed fish
• Use a properly sized filter — ideally a bog filter
• Use plants and bacteria as the primary nutrient processors

My Pond Formulas Blueprint goes through the full bog filter design method if you want to check that out.

How to fix green water

Identify the source and correct the imbalance:

• Reduce feeding (or stop temporarily)
• Reduce fish numbers
• Increase filter size or efficiency
• Add more plants
• Add beneficial bacteria
• Divert nutrient-rich runoff
• Be patient with new ponds
• Use a flocculant if you really want a quick clear, knowing the nutrient remains

If you want the “easy way,” a UV light (Amazon link) will kill the algae as it passes through — just remember this hides the nutrient problem rather than fixing it.

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Brown Water (Tannins)

Brown or “tea-coloured” water is from organic material breaking down — leaves, wood, gum nuts, seed pods, anything plant-based. It’s the same compound that makes black tea brown.

The good news?
Tannins aren’t harmful.

Where I live, the natural rivers are always tea-stained because the catchments are full of forest litter.

Does brown water affect pH?

Tannins make the water slightly acidic. That’s usually not a problem because:

• Many ponds naturally contain minerals like calcium and magnesium that buffer the pH
• You can add buffering minerals if needed
• Fish and bacteria adapt surprisingly well unless the pond is overstocked

Some algae struggle in tannin-rich water because:

• Light doesn’t penetrate as easily
• Tannins bind nutrients like iron and phosphorus

So if you can live with the look, tannins actually have benefits.

How to remove tannins

If brown water drives you mad, here are your options:

• Activated carbon (works well but needs replacing when saturated)
• Water changes to dilute the colour
• Regular removal of leaves and organic debris
• Ozone units (very effective but must be set up correctly)

I made a video about ozone, but I don’t claim to be an expert — so do your research before going down that path.

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String Algae (Filamentous Algae)

String algae is its own beast. It isn’t one species — it’s a whole group: Cladophora, Spirogyra, Rhizoclonium and more. Some love sun and nutrients. Others grow happily in shade, in clear water, with barely any nutrient at all.

Why does string algae grow even in good ponds?

A few reasons:

• It only needs 1% of full sunlight to photosynthesise
• It feeds off the biofilm on every rock and plant
• Temperature swings (spring/autumn) trigger growth spurts
• Plants aren’t pulling nutrients fast during early spring or autumn die-back

Even in shaded ponds, reflected light is enough for some species.

String algae isn’t always bad

A thin coating is actually beneficial. It absorbs nutrients and feeds tiny organisms in the pond.

It becomes a problem only when it:

• Clogs intakes
• Smothers plants
• Blocks water flow
• Forms big mats

What I’ve tried over the years

Pretty much everything:

• Manual pulling (the most satisfying)
• Hydrogen peroxide
• Sodium percarbonate
• Beneficial bacteria
• Filter cleaning
• Phosphate binders
• Diatoms
• Zeolite
• Copper-based treatments

Click here for my favourite options.

And the funny thing?
Half the time it all disappears across all treatments — even the control jar.

String algae always keeps me guessing.

The one upside

Ponds with string algae almost always have crystal-clear water.
The algae traps fine sediments, and when you remove it, you’re physically removing nutrients and waste.

Practical approach

• Remove the bulk when it builds up
• Stop feeding fish temporarily
• Improve circulation and filtration
• Let nature handle the rest

It’s not an exact science, but it works.

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Cloudy or Muddy Water

Cloudy water, in my experience, is almost always clay particles suspended in the water column.

Some people report bacterial blooms, but they’re rare and usually short-lived. I’ve never had one in any of my ponds.

Why clay stays suspended

Clay particles:

• Are extremely fine
• Carry charged ions
• Don’t clump together
• Don’t sink
• Don’t get trapped in most filters

So they just hang in the water.

New ponds

When I build a new pond, I use rock and pebble — and I don’t rinse it. When the pump fires up, the water looks like chocolate milk.

I use a flocculant to get things settled.
Not perfect (the nutrients remain), but it fixes the initial cloudiness.

Clay-lined or natural ponds

This is different. Clay constantly gets stirred up, so flocculant isn’t economical.

Instead, rely on biology:

• Plants
• Roots
• Stems
• Leaves
• Biofilm

These give the clay particles something to cling to and help the pond stabilise.

Deep natural ponds can take years — even generations — before needing dredging. Adding aeration or bacteria products helps slow down sludge buildup too.

In short…

Cloudy water is almost always a phase.
Once the plants and biology mature, clarity improves naturally.

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Final Thoughts

Ponds go through phases. They tell you exactly what’s happening if you know what to look for. And most of the time, the “messy” bits are just nature doing its thing.

A healthy pond isn’t always a perfect-looking pond.

If you want to dive deeper into how ponds work — bog filters, sizing your system, designing a low-maintenance pond, or building your first pond — I’ve got plenty of tools and calculators here on the site.

Thanks for reading, and happy ponding!

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