string algae

String Algae

It is the case that every spring, as temperatures rise and daylight hours increase, an algal bloom occurs in almost everyone’s koi pond.  Dreaded algal blooms, like green water that make koi disappear from view or unsightly string algae that seems to pop up from nowhere, seem to be an inescapable fact of pond life.  Being able to effectively control pond algae is going to be crucial.

There is a lot that happens during the transition from winter to spring.  One thing that many koi owners experience is an increase in disease.  This increase is, in part, linked to the fact that little beasties like parasites are doing well in the increasing water temperatures but a koi’s immune system is still coming back into full strength so there is a period in the spring when they are more vulnerable.

Additionally, the increase in temperatures, sunlight and available nutrients from dead and decaying plant material and fish waste act to fuel algal growth and a bloom occurs.  But what about the beneficial bacteria in the filter media?  

They are still there but, like a koi’s immune system, are not at full strength yet (more on beneficial bacteria later).  Algae are pretty simple as living things go.  They need sunlight, carbon dioxide (given off from the gills of respiring koi and atmospheric CO2) and nutrients (nitrates, phosphates, ammonia  etc.). 

There is more to the story though when it comes to what algae need and how well they will do.  Algae do well when pH is on the higher side and this is because certain nutrients are more readily assimilated by the algae under these conditions. 

Algae also thrive under conditions of stagnant water or decreased water flow (you’ve probably noticed that stagnant ponds tend to harbor a lot of algae).  Different algae behave differently though- read on for the characteristics of the most problematic types.

Two main types of algae

1) Phytoplanktonic (free floating) which includes types that cause “green water” or “pea soup” water conditions. This type of algae may be the most common to afflict pond owners.  Spring is often when ponds turn into what appears to be a large vat of pea soup-certainly not how koi keepers want their ponds to look. 

Besides being a spring bloom occurrence, this single celled algae is often associated with newly established ponds as well due to the fact that the filter hasn’t had time to establish a sufficiently large bacterial population yet.


Green Water Algae

Green Water Algae


2) Benthic (attached) which includes “string algae” or “horsehair algae”, “water net” and “blanket weed”. String algae can be a tough one.  This algae can remain dormant for years in a dried state until introduced to water after which it will thrive. 

Another problem is that when you manually remove it from your pond (which is the best way) the action of removing it causes it to release reproductive spores into the water and the cycle starts again. 

As string algae tends to produce a good deal of dissolved oxygen it tends to aggregate bubbles tangled in its “hair” and before long a big, unsightly mat of the stuff floats to the surface further reducing the beauty of your pond. 

Of course, something that produces dissolved oxygen in your pond is a good thing, right?  Yes, up until the point that it dies, sinks to the bottom and is broken down by bacteria that use oxygen to do so thereby depleting your pond of dissolved oxygen.


string algae mat

String Algae Mat



Algae Box-what algae needs to grow


The following is a list of ways to prevent algal growth in your pond.


Sunlight is a big component that is necessary for algae to thrive so by shading your pond in some way you can effectively reduce some of the potential algae fuel entering your pond.  One way you can do this is the old fashioned way- trees.  Try planting trees that provide canopy overhead near your pond. 

Besides aerial shade there is also the aquatic kind.  Pond owners have, for a long time, installed aquatic plants like lilies in their pond to not only create shade but they make your pond more aesthetically appealing. 

The “magic” number to shoot for when it comes to aquatic plant coverage is 60-70% surface coverage.  Another way to reduce light penetration is through non-toxic coloring agents (dyes) that essentially tint your water a certain color and reduce the available light in your pond.  

This has become a “go-to” solution for plenty of pond owners.  They typically last a long time and you have a few dye colors to choose.  If you reduce the sunlight penetration you reduce algae’s ability to thrive!  Here are few good dye choices to check out.

Reduce Nutrient Loads

Nutrients like nitrates and phosphates are key to algal growth so by reducing and nullifying these components you can severely limit algae’s ability to grow.  This is achieved by not overfeeding your koi, by keeping close tabs on water chemistry and making adjustments as needed. 

Make sure your pond isn’t subject to fertilizer runoff as that will often carry a lot of phosphates.  Be sure you have plenty of filtration and beneficial bacteria to assimilate nutrients etc.  You may also need to perform several water changes in an effort to reduce nutrient loads. 

If this is the case be sure that the water changes you do are gradual to ensure that your pond doesn’t undergo a significant pH swing as this may cause harm to your koi.  For more on how biological cycles work see this article.

Adding Salt

Salt seems to be a go-to remedy for a lot of things in the world of koi keeping and it turns out that it can help control algae blooms as well.  There is a caveat with using salt to combat algae in your pond though and that relates to the fact that high enough salinities will also harm or kill your aquatic plants. 

For example, common plants like water hyacinth and lotus will begin to die back at 0.10% whereas water lily won’t die off until 0.5% and to deal with algae effectively you will want to shoot for 0.25 to 0.30%.  You will have to determine if salt makes sense for your algae problems based on your resident species of aquatic plants.  

A good way to determine the right amount of salt is to the use koi-care’s online calculator and once added a simple way to check your salinity is with a commonly used tool called a refractometer.

UV Sterilizers    

One of the most effective ways of combating single celled algae like that which causes “green water” is an in-line UV sterilizer as part of your filtration system.  Its an excellent and non-invasive way of dealing with certain types of algae (and harmful bacteria for that matter) that can easily be added to your existing piping.  So how does UV light kill algae?  UV is a powerful kind of light energy and it effectively penetrates the cell membrane on the outside of the algae cell and hits the areas of the algae where the DNA is (nucleus and chloroplast).  Once this happens, the DNA gets so disrupted by the UV that the algae cell cannot reproduce.

How UV kills algae explained 

UV Clarifiers are also an option  if you are just targeting free floating algae but its less powerful (algae requires less powerful UV to be killed) so if you are going have a UV system you might as well have one that is going to kill other microbes and bacteria, too.  For most ponds a 30 watt system should suffice but be sure that the light you are getting is rated for the number of gallons you have.

Beneficial Bacteria

Besides UV sterilizers one of best things you can do for your pond is adding additional beneficial bacteria.  This is especially true during spring time when your filter media is not ramped up like it would be in summer. 

One of the more popular products on the market for getting your bacteria populations up is called Microbe-Lift PL and they even have seasonal “blends” depending on your needs (and season).  

Its generally a good idea to give your bacteria a boost from time to time but when it comes to algae you may find yourself in a cycle where the algae dies (either naturally or via algaecides), it decomposes on the bottom and causes high levels of nutrients like ammonia and nitrates and those nutrients then fuel the next generation of algae. 

Adding the beneficial bacteria will allow the nutrients to be assimilated before they become available for more algae thereby starving out future algal growth.  

Koi Clay

Koi clay is one of those additions to your koi pond that can only help.   This “stuff” is a natural way to add a lot of great minerals to your system and koi seem to love it.  As a side effect it has been reported to really be effective at inhibiting and killing string algae. 

It is a calcium bentonite clay and when added to your pond it will temporarily cloud it up.  It clears up in a day and will have added lots of beneficial minerals and removed toxins.  It is said that Kentucky produces so many great race horses because they eat the grass growing in Kentucky’s particularly calcium-rich soil.

Similarly, Japan’s koi might be so revered because of the clay rich ponds in which they are raised.  There’s lot of great koi clays on the market but you want ones that don’t remain cloudy for extended periods.


Barley Straw


One algae treatment you may have heard about but is perhaps a bit unexpected is barley straw.  You can get it as raw barley straw or its extract.  This treatment for green water can take up to 30 days to really get going and the results can be hit or miss.  

Some speculate that the barley straw works by breaking down and releasing a toxin that prevents algae while others suggest that the break down process produces hydrogen peroxide which creates a poor environment for algal growth. 

According to Rutgers University no one actually knows how barley straw prevents algal growth but its important to note that it prevents algae, it doesn’t kill existing algae so it shouldn’t be used as an algaecide.  This treatment is more effective on free floating algae as opposed to string algae and is typically used in the spring time. 


Chemicals for Treating Algae


Most algaecides can be placed into one of three categories: potassium permanganate-based, copper-based and simazine-based.  Simazine is a commonly used algaecide.  The way this chemical works is by disrupting the photosynthetic process and thereby killing the algae.  Caution should be used with this chemical as it can harm or stunt the growth of your aquatic plants (as they use photosynthesis, too).   

Potassium Permanganate

Use potassium permanganate with caution.  Not only is it used for parasites like costia but will also readily kill algae however you need to monitor the pond after you add it.  The dose should be around 1 teaspoon per 1000 gallons to start but you may end up adding more or doing more treatments based on your needs and how much algae you have. 

You will need to double up on your pond aeration as you will see a lot of your fish come to the surface and gasp to get air.  Keep up the treatment for about 8 hours and make sure the treated pond water doesn’t go through your filter media as your beneficial bacteria will be killed off. 

Potassium permanganate will get used up as it kills the algae and parasites etc. so you won’t have to do a big water change as you would if you added a lot of salt.  It would be very helpful though to vacuum the bottom of your pond after the treatment is over and your fish aren’t showing signs of stress.

Algae Fix

Algae fix can be placed in the “copper based” category.  Most copper based algaecides are in the form of chelated copper (which lasts longer than other forms).  It can be effective as an algaecide because it disrupts algae cell metabolism however as with most treatments there are some precautions to be aware of. 

Vascular plants like water clover won’t be affected by the copper but other plants that derive nutrients from the water itself may be negatively affected.  The other thing to consider is copper’s affect on invertebrate organisms like snails and crayfish.  Because most invertebrates have copper-based hemolymph (blood) copper-based treatments will harm or kill these organisms.

Green Clean

One of the newer products on the market, “Green Clean“, kills algae via oxidation and results are very rapid.  There is no residue and it is not copper based.  Though it is advertised as a “broad spectrum” algaecide users have reported that it is best for string algae and not ideal for green water (free floating) algae.  The company provides a helpful demonstration video here. All the Green Clean material is in the first 4 minutes.



Accu Clear

This solution is in the family of treatments that cause green water algae to flocculate (suspended materials form small clusters and sink to the bottom).  The idea is that your filter will take care of the rest but as mentioned previously if you don’t vacuum the bottom afterwards it’s a good idea to pump up your beneficial bacteria populations to handle the excess nutrient fallout from the decay of the algae.  


Every pond owner will, at some point, have to deal with algae.  Typically people will struggle with it during the springtime when temperatures rise but fortunately there are plenty of go-to solutions. 

There are some that are broad spectrum and some that are will target one kind of algae or another.  The solution that is right for you is the one that meets your particular needs.  There are plenty of algaecide chemicals available on the market today and a lot of pond owners will attest to their effectiveness.

However, if you find yourself overrun by algae and don’t know where to start try some of the solutions found under “Prevention” in this article first before adding chemicals. You may have to go the chemical route though and if you do be sure to couple those treatments with some of the preventative measures talked about or you may find yourself in the same situation before too long.

Main Points

  1. Shade is a great way to reduce one of the components algae needs to grow: sunlight.
  2. Reducing excess nutrients is another effective way to minimize algae food.
  3. Salt can be effective at keeping algae at bay. Algae likes fresh water so a little salt goes a long way.
  4. Green water algae can’t stand up to UV sterilizers.  The powerful light energy destroys the cells.
  5. Beneficial bacteria are your natural defense against a build up of nutrients that can potentially fuel algal growth.
  6. String algae can be effectively treated with a time-honored application of koi clay.  Koi actually love the stuff too!
  7. Barley straw has been shown to be an effective prevention treatment for future algae episodes.
  8. There’s lot of other ways to attack algae too.  Algae Fix goes after algae cell metabolism, Green Clean is a very effective oxidizer and AccuClear is a flocculator (grabs suspended materials like green water algae).