Have you ever seen green gunk floating at the top of your waterfalls or clinging to your rocks? That’s probably string algae, an organism that commonly hangs out around water features and feeds on excess nutrients in the water.
We might not like how it looks, but string algae is a natural, normal, expected, and beneficial part of a balanced ecosystem pond.
You’ll likely always have a little bit of string algae in your pond. But that doesn’t mean we have to let it take over. You can take a few simple steps to keep this slimy green gunk at a minimum.
At a Glance: How to Get Rid of String Algae
- Make sure that what you have is string algae, not single-cell green water algae.
- Understand that a little bit of string algae is normal in a healthy pond.
- Starve your algae! Add a variety of cool pond plants to use up all of the excess nutrients so there is very little left for the string algae.
- Remove excess algae by hand or with a contact algaecide.
What Causes String Algae?
The green stuff growing in your pond goes by a lot of names: stringy algae, filamentous algae, and fuzzy-type algae, just to name a few.
For the sake of simplicity, we’ll refer to all these varieties as string algae, which is basically most kinds of green pond algae you can physically grab a hold of. (This differs from the single-cell green water algae, which turns your water green and requires different treatment.)
String algae is a natural, normal, expected and beneficial part of a balanced ecosystem pond.
String algae can grow just about anywhere in your pond. It can grow on the surface; it can grow down deep. It can grow in a pond that’s full-shade; it can grow in a pond that’s full-sun. It can float on the water or cling to the rocks.
String algae arrives in your pond via airborne spores. It then feeds on excess nutrients, specifically nitrates, that build up in the water during biofiltration.
Biofiltration is the process through which beneficial bacteria keep your water clear and healthy. Leaves, pollen, and fish waste all contribute to excess nutrients in your pond. These nutrients break down and create dangerous ammonia. Beneficial bacteria and enzymes then break down the ammonia, keeping your fish happy and healthy. This process leads to the creation of nitrites, which other bacteria then break down into string algae’s favorite food: nitrates.
Is String Algae Harmful?
The most common questions we hear about string algae are “Is it normal?” and “Is it safe?” The answer to both is yes. String algae is completely normal, and it almost never poses a danger to fish. We see these types of algae in pretty much every ecosystem koi and goldfish pond we build.
String algae isn’t necessarily bad. In fact, it helps keep your pond ecosystem balanced by removing nitrates and phosphates from the water. It also provides a snack for fish and other aquatic life.
String algae outbreaks come and go throughout the year, and they’re usually not anything to worry about. While some sources warn of algae suffocating fish, you would really need to have an extreme case – and lots of other issues with your pond – to create any real danger. (And you’re much more likely to suffocate your fish with liquid algaecide than you are by letting string algae grow!)
Still, we know string algae isn’t the prettiest thing to look at. Luckily we can work with our pond’s natural biology to prevent it from growing out of control.
How Do I Get Rid of String Algae?
1. Add More Plants
The best way to get rid of string algae in your pond is to add more plants. Plants remove excess nitrates, starving string algae before it can run amuck.
What kind of plants remove algae? All of them! And the wider the variety you have, the better.
Each kind of plant you add to your pond pulls out a different kind of nitrate. String algae, on the other hand, will feed on whatever’s available. So add the largest variety of plants possible to remove lots of different kinds of nitrates.
Variety comes in many forms: leaf shape and color, flower color, bloom time, hardy vs. tropical, etc. The trick is to figure out what your pond is missing, and add it. So if all your plants have green leaves, introduce some that have red leaves. Or if you only have hardy plants, try some tropicals. Don’t have any floating plants? Throw in a few water hyacinths or water lettuce, and let those long roots suck up all the nutrients they can eat.
Not only does having a variety of plants reduce string algae; it makes your pond look awesome too!
Regardless of what plants you add, you’ll know they’re doing their jobs if they grow big leaves and flowers – meaning they’re taking in lots of nutrients – and the area downstream from them has minimal string algae. You won’t see instant results, but you will see significantly less string algae in the long run.
You will always have some string algae in the pond. This is natural and normal. Feel free to leave it alone, or use the treatments listed below to add some final polish.
If you try other treatments, make sure to add plants first. While lots of products will kill algae, adding plants is the best way to remove excess nitrates from the water and treat the problem at its source.
2. SAB or EcoBlast
The first product we recommend for getting rid of string algae is a contact powder like SAB or EcoBlast (available at Splash and through our Online Store).
These products kill string algae on contact and carry fewer risks than liquid algaecides. Still, we recommend treating the pond in sections – waiting a few days between each treatment – if you have a large amount of algae.
How to Treat String Algae with SAB or EcoBlast:
- Turn off the waterfall and place an aerator in the pond.
- Sprinkle the powder directly on the algae. You want to use enough that you can see it on the algae, but not so much that the algae looks like a sugar-dipped strawberry.
- The powder should take effect almost immediately. The algae will start to bubble and look bleached.
- Wait at least 10 minutes to give the product time to work before turning the pump back on.
- Use a net or your hands to skim out the algae as it breaks away from your rocks. Any dead algae you let decay in your waterfall will become food for new algae, undoing all the work you’ve just done. Empty the skimmer frequently over the next few days to remove any algae that ends up inside it.
- Wait a couple days to do a second application if needed.
Contact algaecides and algae-control products often work better in warm weather, so you might have to wait longer to see results in early spring than you would in the summer.
(By the way – EcoBlast and SAB will both work just fine against string algae. EcoBlast is slightly less expensive, but we generally prefer SAB because it includes beneficial bacteria and phosphate binders that help improve the overall health of the pond.)
3. Autodoser with Clear for Ponds
The Automatic Dosing System for Ponds automatically adds a constant drip of water treatments to your pond, eliminating the guesswork and routine of adding products manually.
If your pond already has an autodoser, try switching out the Maintain pouch for the Clear pouch every two weeks or so during the season. (If you have an XT system with the controller add-on, use both treatments simultaneously). Clear contains a small amount of algaecide, so make sure to follow the dosing instructions on the container.
Only run your autodoser when temperatures are above 40 degrees. You’ll need to shut down the autodoser in winter to avoid damage to the system.
If your pond does not have an autodoser, they’re easy to install. Contact our Service Team, or find one at our Online Store.
4. Remove it Manually
There’s something satisfying about pulling a handful algae out of your waterfall.
If you’re up to the task, removing algae by hand is sometimes the easiest and most effective way to deal with it. Just don’t get too overzealous with cleaning your pond rocks; you want to maintain as much of the natural ecosystem as possible. That means no cleaning products, and no power washers.
5. Improve Your Filtration
Some ponds have more nutrients than plants can realistically consume. If you have persistent algae problems that don’t clear up with plants or other treatments, consider installing a larger skimmer and/or biofilter.
You can also reduce the pond’s nutrient load by reducing the amount of food you throw in, keeping fish numbers low and installing a leaf net in the fall. Check out our guide to clear water for more tips.
6. Use Barley Straw Extract
Barley straw extract creates water conditions that deter new string algae growth. It won’t kill existing algae, and it doesn’t work for every pond, but it’s safe and inexpensive if you want to give it a try.
Extract works better than barley bales and pellets, which can rot in the pond.
7. Install an IonGen
Some pond owners like to install IonGens on their pond piping for constant string algae deterrence. These devices release copper ions into the water through a probe, creating conditions that string algae don’t like.
They’re a powerful tool if you don’t mind the up-front cost, plus the cost of replacing the probe every one to three years.
Contact our Service Team if you’d like us to install an IonGen in your waterfall, or pick one up from our Online Store for DIY installation.
8. Leave It Alone
String algae is a normal part of your pond’s ecosystem. If it’s not excessive, and it doesn’t bother you, you can leave it be.
Treatments to Avoid
Avoid adding liquid algaecide to any water feature that contains fish. Using algaecide depletes the water of oxygen, leading to fish suffocation when dosed incorrectly. If you must use algaecide, always keep your waterfall running, install a supplemental aerator, and err on the side of under-dosing. Never use algaecide on hot days.
You should also avoid UV filters. A UV won’t hurt your pond life, but it isn’t designed to deal with this type of algae. In order for a UV to work, the algae needs to pass through the device and receive direct exposure to the algae-killing light. String algae usually sticks to your rocks – meaning it never reaches the UV.
