Green Water, String, And Scum: What Kind Of Algae Is In Your Pond?
Staring at green water and wondering what went wrong? Pulling handfuls of slimy green strings off your pump?
Not sure if the stuff in your pond is harmless or a real problem?
In this post, you’ll learn how to tell the common types of pond algae apart, what causes each one, and what actually clears it.
By the end, you’ll know which algae you are looking at and what to do about it here in Maryland.
What Are The Main Types Of Pond Algae?
The most common types of pond algae backyard owners deal with are:
- Green water, a planktonic algae that turns the whole pond pea-soup green.
- String algae is a filamentous algae that grows in long green threads and mats.
- Blue-green algae, which are actually cyanobacteria, can be toxic.
- Brown algae, a harmless film of diatoms, are common in new ponds.
- Chara is a gritty macroalga that resembles a submerged plant.
Green Water (Planktonic Algae)
Green water is the type most people picture when they think of pond algae.
It is the most common algae complaint we hear, especially in spring and early summer.
It comes from planktonic algae, which are microscopic, free-floating cells suspended throughout the water.
When they bloom, they tint the whole pond a cloudy pea-soup green, sometimes shifting toward brown or olive.
In small amounts, this algae is normal, and it even feeds the bottom of your pond’s food chain.
A heavy bloom is the problem because, as it dies, it can pull oxygen out of the water at night, putting your fish at risk.
The bloom is usually a sign of too many nutrients and too much sunlight, and it often shows up after a cleanout or in a young pond still finding its balance.
You cannot rake green water out, since the cells are far too small to catch.
The real fix is clearing the water properly and controlling the nutrients feeding it.
At Premier Ponds, we use an IonGen system that targets algae at the source as our algae solution, and clearing green water is part of the water treatments we start each spring.
String Algae (Filamentous Algae)
String algae is the long, green, hair-like growth that clings to rocks, liners, and the face of your waterfall.
It starts along the bottom in shallow areas, then floats up in tangled mats once it traps the oxygen it produces.
You will know it the moment you reach in and pull out a handful of wet, stringy green threads.
A little is harmless, but heavy mats clog pumps, skimmers, and the filters you then have to clean out, and they take on the look of pond scum.
String algae feeds on the same excess nutrients as green water, so fallen leaves, fish waste, and lawn runoff all make it worse.
Because it is large enough to grab, we remove it by hand and power wash it off waterfalls and streams during maintenance visits.
Left alone, string algae can hang on through the cooler months, so staying ahead of it matters.
Cutting the nutrients that feed it, with good filtration, netting, and aeration that keeps the water moving and oxygenated, is what keeps it from growing back.
Blue-Green Algae (Cyanobacteria)
Blue-green algae is the one type you should never ignore.
Despite the name, it is not a true algae but a bacterium called cyanobacteria, and it can produce harmful toxins.
It usually appears as a bright bluish-green, turquoise, or paint-like scum on the surface and often carries a foul, swampy smell.
It thrives in warm, stagnant, low-oxygen water, which is why it tends to appear during the hottest part of summer.
Those toxins can make pets, wildlife, and people sick, so keep children and dogs away from the water if you suspect it.
This is not a do-it-yourself fix because killing it too quickly can release more toxins and crash the oxygen levels in your pond.
Steady aeration and circulation make your pond far less welcoming in the first place.
If you think you have blue-green algae, the safest move is to call us so it gets handled correctly.
Brown Algae (Diatoms)
Brown algae are usually the least worrying type on this list.
It appears as a thin brown or rusty film on rocks, the liner, and equipment, and it is composed of tiny organisms called diatoms.
You will most often see it in brand-new ponds or right after a water change, while the pond is still maturing.
In most cases, it clears on its own as the beneficial bacteria that keep a pond balanced establish, and the ecosystem settles.
Some fish will even graze on it, so it rarely calls for any treatment at all.
A brown film is a sign that your pond is still young, not that you did something wrong.
Our ponds run an Aquascape 2500 BioFalls and a skimmer that builds that biological balance, which keeps a brown film from lingering.
Chara (Macroalgae)
Chara is the sneaky one because it looks like a submerged plant rather than algae.
It grows in gritty, branched mats along the bottom, has a rough, calcified texture, and gives off a skunky smell when crushed.
A close cousin called Nitella looks similar but is softer and odorless, and tends to show up in softer water.
People mistake Chara for a harmless pond weed and leave it alone, but it spreads fast and can carpet the entire bottom.
Like string algae, it grows from the bottom up and then releases into floating mats.
It is most common in hard water and can be easy to miss until it has already spread.
Removing it by hand during a cleaning is the most reliable way to keep it from taking over.
How We Keep Your Pond Algae-Free
A little algae is part of every healthy pond, so the goal is balance, not a sterile, lifeless pool.
Our approach starts with the equipment that handles the daily work, including an Aquascape 2500 BioFalls for biological filtration, a skimmer that removes debris before it rots, and aeration that raises dissolved oxygen levels through the hot summer months.
On top of that, we install IonGen systems as our algae solution and begin water treatments each spring as part of our seasonal maintenance.
We also add pond netting to cut down the leaves and debris that feed algae in the first place.
When algae does get ahead of you, our cleaning visits handle the filter cleaning, power washing, and hand removal that bring a pond back to clear.
The result is water clear enough to actually see your fish through, season after season.
For homeowners across Maryland, Washington DC, and Northern Virginia, we have built more than 2,000 ponds and handled over 8,000 cleanings since 2001 as an Aquascape Certified contractor.
Stop Guessing At The Green With Premier Ponds
We keep ponds clear by matching the fix to the algae, from IonGen systems and aeration to hands-on cleaning and the spring water treatments that head off blooms before they start. As an Aquascape Certified contractor with more than 2,000 ponds and 8,000 cleanings under our belt, we can tell you which type you are dealing with and how to clear it.
Fill out our contact form today or give us a call, and we can fold algae control into a seasonal maintenance schedule that starts with a spring cleaning and water treatment. That keeps your water clear from the first warm week instead of you fighting it all summer.
