Algae is the visual scourge of koi keepers everywhere. It quite literally seems to come out of nowhere. At bedtime, the water is crystal clear. The next morning it is green and leafy.

Algae, other than keeping your koi hidden from view and choking your filters, does have its advantages. It releases oxygen and can be a nice snack for a peckish koi. Of course, that’s only the case when the algae is just a nice carpet on your liner and not completely out of control.

Types of Algae

There are two types of algae that afflict koi keepers ponds: filamentous algae and planktonic algae.

Filamentous Algae

String, mat, blanket: it goes by a number of names, but this type of algae is an absolute nuisance. It is a single-celled organism that grows into long strands (or chains) starting on the pond’s bottom. The strands break off then float to the surface to create the blanket effect that gives it one of its monikers. Filamentous algae clogs filters and water features, and it can prove difficult to get rid of when it gains a foothold.

Planktonic Algae

Planktonic Algae is the single-celled algae that turns your pond green and makes seeing your living jewels difficult. At night, algae will soak up oxygen from the water to feed itself. If enough of this algae is present, it can cause an oxygen crash that can be fatal to any wildlife in your pond that relies on oxygen to survive.

Prevent Algae Overgrowth

As always, prevention is worth more than its fair share of cure. Keeping Nature from being, well, Nature, is a constant struggle. You can’t stop those natural processes that make your pond and koi what they are; it’s more about managing them so that curing the problem isn’t necessary.

So what steps can you take to keep algae and blooms out of your pond? It’s mostly about starving them, honestly.


It bears repeating that proper and ample filtration is the first and most important aspect of happy koi and a healthy pond. The more potentially harmful organic matter or debris that is removed from the water, the less likely it is that any number of issues will arise in your pond.

Proper mechanical and biological filtration (as well as ample amounts of oxygenation) can go a long way to help prevent algae from blooming. It is not a guarantee, but it is a great start.


One of the easiest means of starving algae is to feed your koi the correct amount. Uneaten food ends up at the bottom of your pond as additional organic material, and once it breaks down, it can be a source of nutrients to algae that aids in its proliferation. Overfeeding will also place a heavier load on your filtration, which can impact overall water quality.

The general guide is not to feed your koi any more than they will eat in five minutes.  After five minutes is up, remove any uneaten food from the water.

It is important to note that feeding requirements change seasonally as the temperatures rise and kois’ dietary needs increase. A full meal in May is not the same as a full meal in late July. Additionally, the number of daily feedings will vary. Aquascape Inc looks at overfeeding in their article How often or how much should I feed my Koi and Pond Fish?

Higher Ground

More than a few pond owners have found to their detriment that building a koi pond at the lowest point in their yard is an invitation for disaster. It is essentially like building a pond at the bottom of a drain. Anything that travels downhill will eventually end at the lowest point.

If your koi pond is in a natural low spot, the runoff will introduce things that you don’t want in your water column. Things like phosphates (from fertilizer), debris, sulfates, nitrates, heavy metals, and pesticides not only lead to poor water quality, but they also provide nutrients that encourage algae growth.

Pond plants

Pond plants fulfill a couple of different positive roles when it comes to keeping algae out of your pond.

As pond plants and algae generally need the same staples to exist, they will compete for nutrients and basics that algae use to bloom.

Pond plants also provide shade. More shade means less light going into the pond, which means less photosynthetic capability for any algae.

Of course, pond plants are a lot nicer to look at than pond scum or green water.

UV Filter

Installing an ultraviolet filter is an additional precaution that can help with single-celled algae.

Water flows from the pond into the sterilizer (or clarifier depending on the speed with which the water is pumped over the UV Bulb), exposing the algae to the UV light. UV light has a higher wavelength, and when algae is exposed to it, it irradiates the cell, causing a mutation that stops the algae from multiplying.

One thing to note is that a UV filter will not combat filamentous algae. It can also prove to be harmful to beneficial bacteria in your pond as it doesn’t discriminate in its exposure.


Algae needs sunlight to grow. In the process of photosynthesis, algae uses the sun’s energy to produce energy. Shade is a great way to limit algae’s ability to create sugars. Algae can exist without direct sunlight, but it won’t thrive.

Natural shade includes elements like trees or floating pond plants, while man-made shade can be constructed out of objects like sail cloths or awnings. If neither natural or man-made shade is available, some koi keepers will turn to dyes. Dyes come in a range of colors: from black to various shades of blue and green. The Pond Guy offers a little insight into which dyes are available and how to use them appropriately.

Lower Phosphates

Phosphates are one of the ingredients  found in fertilizer. It is a chemical combination of phosphorus and oxygen. And yes, algae loves it.

Where do phosphates come from? Things like degraded fish food, tap water (yes, it’s in there too to help with pipe maintenance) and even bird droppings can add to the phosphate content in a pond. There will always be some phosphates in water, and it’s not always bad as it helps with normal aquatic plant growth.

Keep Debris Out

Debris that ends up in your pond will eventually decay, which leads to extra nutrients that algae thrive on.

Your mechanical and biological filtration work in coordination with one another to maintain the delicate balance in your pond. When they are maintained and functioning optimally they can perform well under sometimes challenging circumstances.

However, even top-of-the-line filtration systems and the best algae-prevention practices can fall short in the face of an aggressive algae outbreak in your pond. At that point, it’s time to cure the problem.

Cure Algae Outbreaks

Once you have an algae problem, you go from defense into attack mode. It is important to remember that although you are getting rid of the algae, you will need to determine and fight the cause of the algae; otherwise, it will come back.

Manual Removal

One means of removing the filamentous pond scum algae is to rake it or vacuum it out. Manual removal can be a time-consuming process, and it won’t completely solve the problem, but it can help to keep the algae manageable until the underlying causes of the bloom are addressed.   


Many koi keepers don’t have the patience to look for natural solutions to get rid of algae once it has become a problem. Instead, they turn to additives to help rid their pond of unsightly and sometimes dangerous algae blooms.

Although some algaecides may work more quickly, they can have negative consequences for the plant and fish life in the pond. Any algaecide should be used as a last resort and administered very carefully.

There are numerous products on the market and the Pond Informer offers a curated list of the Best Pond Algaecide and Algae Killers on the market.

Barley Straw

Barley straw is a more natural way of combating algae in your pond. It works by releasing small amounts of hydrogen peroxide into the water column as it degrades. While barley straw is a more natural way of addressing algae blooms, the trade off is that it takes longer to work than some of the additives that are introduced directly into the water.

Algae Eaters

Whether added as a cure or a preventative measure, there are a number of creatures that can aid in keeping the number of algae down in a pond. Snails, tadpoles, hi-fin banded sharks and even catfish can help to keep algae in check. However, don’t expect them to act like vacuums and suck up all the algae instantly: they eat at a relatively slow rate.

Although algae can be an issue for pond keepers and their koi wards, it is not an insurmountable problem. Algae can be managed effectively with a little elbow grease and some pointed attention.

3 responses

  1. Alisa welch :

    Enjoyed the article, I’m a new by to a water feature, as your article said: I woke up this morning with grassy water. I have no fish , the tubs (2 plastic), one stainless steel water trough, all rather small with a pump; pumping water in a wheelbarrow. Water flows down the 2 plastic ponds into tank. The tank only is lined, sand on bottom, river rock and the a few large stones.
    All this to ask- my brother said to add bleach; is that good? I want to buy shiners to put in tank hoping they will feed off algae. Can you help me.

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