How To Remove String Algae From Your Pond

Algae in Koi Ponds & Watergardens

Most koi keepers problems with algae start in early Spring, when pond temperatures begin to rise; this is because the nutrients required for the algae to grow come to life. This also explains why you typically do not have algae problems in the colder Winter months, During this time your pond slows down dramatically, with the koi being less active, the filter more dormant thus reducing the nutrient content of the pond.

So how can you get rid of algae in your pond, what causes it and what is the best way to both treat it and prevent it?

The two main types of Algae in your pond

While there are thousands of species of algae int he world, the two main types inhibiting our koi ponds are planktonic algae & string algae

Planktonic Algae makes the water look like ‘pea soup’, and you only see it in a new pond as the water starts to mature or if you don’t use a UV sterilizer on your pond. This type of algae generally isn’t a problem if you are properly equipped. A properlty sized UV can remove it without too much trouble, which enables you to have clear water as soon as the filter has dealt with the small particles of algae.

String Algae, or Blanket Weed, is far more messy and can cause pond keepers a real headache throughout the season, because of it’s make-up and the fact it can really take hold of your pond, giving your filter a hard time and making your job harder.  UV filters cannot prevent this filamentous algae because it is not free-flowing, and does not pass through the pond pump; in fact, it is more likely to block your pump and restrict your water flow, which is obviously not ideal.

The bad news about string algae is that you may never totally rid your pond of it.   Some people never get it at all but many get it at the same time each and every year no matter what they do.  Not to worry though, measures can be put into place to help you prevent string weed, and remove it from your pond if it has already taken hold.


To prevent String Algae, follow these expert tips:

  • Remove any detritus (dead/rotten leaves) from the pond BEFORE the Wintertime.  
  • Cover your pond before the leaves start to fall to prevent further contamination of dead leaves.
  • Restrict sunlight.  Because algae is live matter and requires photosynthesis to grow and thrive, lack of sun will in turn restrict blanket weed’s ability to grow.  A shade or pergola is ideal for this job, and still keeps the aesthetics of the pond looking good.
  • Test your Nitrate level regularly.
  • Lower your Nitrate level. Regular water changes can help reduce nitrates providing less nutrients for algae to grow.  Be sure to test the Nitrate levels in your source water to make sure you are not adding to the problem. Some source water may contain high levels of nitrates.
  • Don't overfeed.  Ensure no food is left over after feeding your koi. Remove and feed less next time.
  • Use lots of aeration in the summer months.  This is both good for the fish and prevents the water sitting dormant.

Adding Plants

Pond plants do an excellent job of out competing algae for Nitrates an Nutrients in the pond as well as providing shade.  Be sure to keep any dead or dying plants/flower debris from accumulation in the pond.  This will be counterproductive and lead to future algae growths.

Manual Removal

Manual removal using a garden rake, pond net or even a shop vac may become necessary if the algae has overtaken your pond or is clogging your pumps.  It is a dirty job and there is no easy way to do it. Taking the above mentioned prevention measures are the better alternative.

Chemical Removal

Chemicals are always a last resort for algae removal and we strongly advise against. Overdosing or overusing algaecides can quickly kill an entire collection of koi.  Unfortunately, we do hear of this happening often.  If you MUST use chemicals be sure to closely monitor your pond during treatment and be prepared to do a large water change to remove the chemicals if necessary.  Be sure you know your exact pond gallons and dose accordingly.  DO NOT OVERDOSE.


The natural balance of your pond does include algae in small amounts;  It can appear at different times of the year, depending on how the season pan out; a benchmark though is from springtime through to autumn, with a winter die-back.  Algae can be kept in check with a little planning and prevention.

We hope that clears up your algae questions, but of course feel free to comment and ask us anything else you want to know about algae and it's prevention or treatment!

  • Apr 05, 2019
  • Category: News
  • Comments: 0
Leave a comment