How to Get Rid of String Algae in Your Pond


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 in ponds, feeding off excess nutrients in the water.

String algae is a natural, normal, expected, and beneficial part of a balanced ecosystem pond.

A properly balanced ecosystem pond will have a little bit of string algae - and that's OK! But sometimes, the stuff can go crazy and overwhelm your pond.   If you understand where it comes from, you can control it.


At a Glance: How to Get Rid of String Algae

  • Make sure that what you have is string algae vs 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.

String Algae: Is It Normal?

Get Rid of String Algae in PondThe 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.

The first question we get a lot about these stringy, fuzzy-type algae is this: Is it normal? The answer is yes. It's totally normal. We see these types of algae is pretty much every ecosystem koi and goldfish pond we build.

The reason you see these types of algae in the pond is because of excess nutrients, specifically excess nitrates.  Where do these nitrates come from?  They are a byproduct of the biofiltration process.  Leaves, pollen, fish waste are all different sources of excess nutrients in your pond.  These break down and create dangerous ammonia.  In the biofilters, beneficial bacteria and enzymes colonize, and they get rid of the ammonia.  This keeps your fish happy.  The byproduct of that process is nitrites. There's another type of beneficial bacteria that eats the nitrites, producing nitrates.

There are so many things that stringy fuzzy algae does to help keep balance in the pond.  One of it’s primary functions is to work alongside your other pond plants to remove nitrates NO3 and phospates PO3 to keep the ecosystem balanced and your fish friends happy.

So the quick science lesson is ammonia gets converted into nitrites, and nitrites get converted into nitrates. Well, what happens to those nitrates in your pond? Basically, your plants absorb them and use those excess nitrates as nutrients to grow: to produce beautiful red leaves, beautiful pink flowers, a narrow leaf, a wide leaf. All of these different characteristics use a different kind of nitrate.

If there's any nitrates left over, then you're going to get stringy, fuzzy, scummy-type algae in the pond. It's totally normal to see these type of algae in a pond, even if the pond is full of plants.  It's actually a sign of a happy healthy pond when you see the green fuzziness on the rocks in the pond.  We're only talking about nitrates here but there are so many other things that stringy fuzzy algae help to keep balanced in the pond.

Treating String Algae with Plants

You know now that string algae is normal. But what if it's too much algae? Our first suggestion is to add more plants.

Let's say you have a plants in your pond that have green leaves, but you don't have any red leaves in the pond.  Add some plants that have red leaves. The red leaf plant will take out a nutrient specific to the red leaves.

Variety is important and comes in many forms, including different leaf shapes and colors, different flower colors, early season grower versus later season, a tropical plant versus a hardy plant.

Not only does having a variety of plants work to reduce string algae, it really makes your pond look awesome too!

If you have a variety of plant characteristics in your pond, they will take out a variety of excess nutrients. If you have enough plants to remove most of the excess nutrients there won't be enough nutrients left over to feed the stringy, fuzzy-type algae. The string algae will be greatly reduced.

Having a wide variety of plants in the pond will greatly reduce string algae while at the same time making the pond look beautiful. So add as many plants as you can to the pond.

Floating plants like water lettuce and water hyacinths have a special talent for taking up excess nutrients thanks to their long roots, which hang down in the water and suck up all of the nitrates floating past them.

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.

Browse plant ideas here.

Other String Algae Treatments

After you've added all the plants you want to add, occasionally you may still have a little bit of excess algae in the pond. In that case, there are some powder-type products available that you can add that act as a contact algaecide.

Keep in mind, though, that if we add algaecide first without adding plants, we're making the problem worse. Dead algae becomes nutrients for next week's algae, so you just end up buying more and more product without actually fixing the root of the issue. You can also remove string algae by hand, thereby preventing the issue of feeding new algae, but manual removal also does nothing to cut back the amount of nitrates in the pond.

At the end of the day, the best way we can recommend to get rid of excess algae is to add beautiful plants to the pond that will get rid of the nutrients. If we get rid of the nutrients, we reduce the amount of stringy, fuzzy-type algae in the pond, and the pond looks beautiful all season long.

Learn More About Pond Algae

3 Things to Know About Pond Algae

Pond Ecosystems: How Skimmers, Biological Filters Keep Water Clear

10 Ways to Keep Pond Water Clear

Thinking About Washing Your Pond Rocks? Don't

What Causes Pond Algae?

How Can I Keep Algae Out of My Pond?

Do UV Lights Destroy Algae?

The Ultimate Guide to Clear Pond Water