Why Is My Koi Pond Foamy? (And How to Fix it)

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Why Is My Koi Pond Water Foamy on the Surface? (updated guide)

pond foam
Foam isn’t harmful by itself, but could be a sign of an underlying problem with water quality. Public domain.

Foam is caused by changes to the surface tension of water, which essentially acts as a thin “skin” between the pond water and air above. Water molecules in your pond are normally attracted to each other, which creates a strong surface barrier and helps prevent air from mixing with water on the surface. However, dissolved organic carbon (DOCs), such as proteins found in fish feed, act as surfactants which reduce the surface tension, allowing air to easily mix with water and create bubbles. In agitated conditions, such as under a waterfall, these bubbles can form in large quantities, eventually sticking together and forming a white foam through their own surface tension.

Even in ponds without any fish, you will still see foam appear, especially if you have waterfalls, fountains, or live in windy conditions. Organic matter increases foaming, but just the agitation from water movement is enough to create small amounts of bubbles. Unless you have fish, most cases of foam will disappear on their own naturally as bubbles break and organic matter is broken down. Although the foam itself is usually harmless, in some cases it can indicate a growing problem with water quality that needs investigation.

Is water foam bad for koi ponds?

what causes foamy water
If you have fish, especially koi, you’ll likely experience foamy water which often caused by excess protein and nutrients. Public domain.

Foam is simply the result of air and water mixing together, and even though it may be an eye-sore, the foam itself is completely harmless to fish.

However, the problem with foam is that it is sometimes a warning sign of an underlying problem with water quality, especially if you’re seeing a lot of foam over a short period of time. Foamy water is the most common in fish ponds as there is a huge abundance of organic matter available, often coming from fish foods and fish waste. If you have waterfalls and fountains in your pond as well, a small amount of foamy water is to be expected and should not cause any problems. If you have a thick layer of white foam, however, this could mean there is too much organic material in your pond and water quality is degrading as a result. Excess organics in ponds can be a side effect of overfeeding, bottom muck (sludge), algae blooms, or as a result of koi spawning when they release hormones into the water. It could also be a product of poor maintenance, inefficient filtration, or even poor quality fish foods.

What causes koi pond foam:

  • Overfeeding (Excess Protein)
  • Poor quality fish food
  • Excess nutrients/fertilizers
  • Algae blooms
  • Poor water quality
  • Changes in water quality
  • Spawning/breeding

If you only have a few bubbles or a small amount of foam, especially if it’s concentrated around a water display, we recommend waiting as it will likely clear on it’s own. If you’re experiencing a huge influx of foamy water, or foam that just won’t go away, testing water quality and taking steps to remedy any problems will help prevent the situation worsening (see treatment section below for more information).

Do koi produce foam when spawning? Is this normal?

why is my koi pond foamy
The hormones koi produce during spawning will cloud pond water and contribute to surface foam. Public domain.

Yes! It is perfectly normal to see foam and bubbles on the water surface when koi spawn, as they’ll be releasing a large amount of organic matter which can cause temporary swings in water chemistry. During spawning koi will release sperm, eggs, and hormones into the water; all of which are comprised mainly of proteins. This increase in organic matter will lower water surface tension and increase the ponds bio-load which will lead to foamy water during the spawning period. There is not much you can do about this, and the pond will begin to balance itself out naturally after the event is over so doesn’t always require any treatment. However, you can supplement with extra beneficial bacteria, optimize filtration, or skim the surface water to help speed up the process if the foam is causing problems.

It should be noted that anti-foam treatments and activated carbon should not be used during spawning as they can interfere with the breeding process. These kind of treatments should only be used after the spawning period to help clear up residue hormones and left over matter which may still be contributing to surface foam.

Koi pond protein skimmers (foam fractionator) – worth it?

protein skimmer to remove pond foam
An example of a small-scale protein skimmer commonly used in salt water aquariums.

Protein skimmers are very popular in the aquarium hobby, often used in salt water tanks as foam can become a real nuisance in high-salinity (salt) conditions. Although they’re very useful in small-scale applications, skimmers designed just for foam removal are less common in the koi hobby, as regular skimmers are often able to perform a similar function with good results. In-fact, protein skimmers designed for large ponds can be extremely expensive and very fiddly to install, so we’d only recommend them for heavy stocked ponds, breeders, or to complement a regular skimmer and filter set-up. The benefit of a protein skimmer is it provides an extra layer of water filtration for better water quality, and collects foam more efficiently which makes cleaning easier if you suffer with heavy foam build-up.

Would we recommend a protein skimmer for your pond? Probably not, as most problems with foam can usually be solved with regular maintenance, good cleaning, and stable water quality. Protein skimmers do have a place in the hobby, being particularly useful for breeders, but they’re often overkill for the everyday pond keeper just looking to remove a small layer of bubbles on the surface water.

If you’d like to solve the mystery of your foamy pond, and also want to stop it coming back, read on below for our step-by-step guide on foamy water treatments.

How to Solve Surface Foam and Fix Foamy Water in Koi Ponds

1) Test & Optimize Pond Water Quality

water quality test kit foamy water
Foam is often a by-product of a substance imbalance, and water test kits can help you identify this.

If you have foamy surface water that isn’t disappearing it could indicate an underlying issue with water quality that needs to be addressed. Although a little foam is natural in ponds, a large amount of rising foam is often due to a build up of dissolved organic matter, which can lead to spikes in ammonia and nitrites.

The best way to tell if you have a water quality problem is to test your pond with a wide range water test kit which provides measurements for things like ammonia, nitrites, pH, and kH (alkalinity). Ammonia and nitrites are highly toxic to fish, and are produced as a by-product of the decomposition process. Organic matter doesn’t just act to reduce surface tension to cause foam, it also produces harmful substances which beneficial bacteria need to break down. Having lots of foam often means you have too much organic material in the pond and your beneficial bacteria populations cannot break them down efficiently. A water test kit can provide these readings and give you an idea of how much matter you may be dealing with, and whether or not you need to manually reduce the bio-load (see steps below).

For more information on testing water quality, the ideal substance levels in ponds, and how to optimize your system, check out our articles below:

2) Skim & Clean to Reduce Organic Matter

pond skimmer to remove foam
Pond skimmers not only remove foam and debris, but can also help with water quality due to their internal filter media.

A great method to remove foam from the surface water and prevent floating debris adding to the organic material in the pond is to install a water skimmer. Although a skimmer won’t be able to remove organic material already on the pond floor, it can help prevent future build-up whilst also keeping the surface free of annoying bubbles. Depending on the size of your pond, you could simply skim the surface daily with a net to remove any floating leaves or twigs, or you can add a dedicated skimmer system to keep on top of things 24/7. Pond skimmers are especially useful for reducing free-swimming organic material as they often come with dedicated filter media which provides extra biological filtration. As most foam is due to dissolved organic particles in the water, especially near the surface, a skimmer is an efficient way to help breakdown these substances alongside your main filter system.

As well as skimming the pond, you’ll also need to address any excess organic material present which may be causing surface foam and water quality problems. Excess sludge (bottom muck) is a common culprit, and this can be reduced using a pond vacuum or heavy duty pond net. After cleaning out any excess muck, you can also supplement the pond with a sludge eating bacteria product to keep sludge levels lower in future.

Finally, you’ll want to make sure you pond is free from algae as they’re packed with organic material which becomes dissolved in the pond when they bloom and die. Algae also greatly contribute to sludge, lower oxygen content, and can become a real nuisance in summer if left unchecked. For our in-depth guide on algae control and treatment methods, check our dedicated guide on this below as well as our guide to types of algae:

3) Switch to a High Quality Koi Food

high quality koi food
Koi food should be low in “filler” content and high in proteins, fats, vitamins, and minerals for the best health and water quality.

Overfeeding fish or using a low quality feed with too much “filler” content can contribute to foamy water as both things can cause organic material to build up in the pond. Overfeeding is one of the more common causes of sudden water foam, as fish feeds contain high concentrations of organic proteins which easily disrupt surface tension. The best way to avoid this is to actively monitor feeding and adjust dosage accordingly if any pellets are left floating on the water. If you’re not able to stay during feeding, you can slowly reduce feeding doses to see if this has an impact on the amount of foam build-up, or alternatively, you could consider investing in an automatic pond feeder which can help with more accurate dosing.

As well as overfeeding, using low quality fish feeds can cause issues with foamy water as the fish will be excreting the “filler” material as waste. When feeding koi you want to make sure you’re using the best quality feed available to you, as this will help reduce pond waste, improve water quality, ensure your fish are in optimal health. Many feeds are packed with ingredients koi don’t need and not many ingredients they actually do, which leads to fish producing more waste which will place more stress of your bio-filtration. The more content within a feed your koi can use, the less waste they’ll produce, and the better your water quality will be in the long term. Top quality feeds will contain a low amount of waste material and be high in aquatic sourced proteins, fats, vitamins, and minerals – things koi actually need in their diet!

For a full list of our recommended summer and winter koi foods, and why they’re good for fish, check the articles below:

4) Consider Small Water Changes

carbon hose attachment for fish ponds
When performing water changes chlorine should be removed, and carbon hose attachments are one of the easiest ways.

While you’re getting to the root cause of the foam you can carry out weekly water changes to improve the appearance of the pond if it’s becoming a problem. Water changes will help remove a certain amount of dissolved organic material, and will also take some of the surface bubbles away, so are a good option while you try to solve the underlying issue.

The problem with water changes is they’ll introduce harmful substances, such as chlorine, to the pond if they’re not carried out properly which can be even more dangerous than excess waste. Water should always be treated before adding to a pond, and you should aim to remove both chlorine and chloramine to be safe (although you can check with your water company to confirm what’s used in your water). Water can be de-chlorinated with a variety of methods, such as activated carbon, carbon catalyst hose attachments, and de-chlorination water treatments. For the easiest water changes, we recommend using a carbon based hose attachment which will neutralize chlorine as it passes through your hose, which means you don’t need to treat the water separately.

If you’re battling a lot of foam we would advise a 10-20% water change every week until the underlying problems are fixed (i.e., excess waste is reduced). For help with de-chlorinating a pond and the best methods, see our articles below:

5) Try an Anti-Foaming Agent (Quick Fix Only)

De-foamers work quickly to remove foamy surface water, but won’t be able to solve the root cause of the problem.

Anti-foaming treatments should only be used as “quick fix” foam removal method, as they cannot resolve the root cause of the problem, so the foam will simply keep coming back! As well as this, chemical treatments should always be a last resort if other methods haven’t worked as they can alter the chemistry of a pond and place unnecessary stress on fish. Anti-foaming treatments, or defoamers, contain substances that either prevent bubbles from forming or disrupt the surface of bubbles causing them to break. Treatments are often oil, water, or powder based, with common defoamers being mineral oil, vegetable oil, or silica.

If you intend to try an anti-forming agent you’ll need to select one which is designed for ponds and be sure to dose correctly to prevent problems with water quality. Defoamers are a very fast method of removing foamy water and preventing it coming back for a short duration, so may be useful for improving the appearance of the pond while you solve any underlying issues. Anti-foaming agents should not be used regularly to combat foam as they can cause issues with water quality, so should only ever be a temporary solution and not a permanent fix.

In terms of pond defoamers we recommend KoiWorx Defoamer as it’s fast acting, safe for fish, and provides long lasting foam removal at a good price.


6 thoughts on “Why Is My Koi Pond Foamy? (And How to Fix it)”

  1. all good reading changing over my filters at moment and using Nexus new one and old Kockney Koi large filter in tandem for a fortnight then leaving the one filter running the Nexus 320 – this is causing my pond to foam due to both filters dropping a lot of water through the outtake pipes and having extra air piped into pond- seem ok thanks for your input…

    • Hi Bernard,

      Yeah, it sounds like your foam is just due to the large amount of surface agitation being caused by the two filters and aerator. If your water quality is within safe parameters, I don’t think there is anything to worry about here.

  2. I struggled with foam. I put in the Carbon and poly. Finally, when I did a water change of about 25%+ the foam problem was no more.


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