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Koi in a mud pond

Koi Mud Ponds

What is a Koi Mud Pond

It’s a pond. With mud. And koi.

The End.

It may be that, and at one point, that is exactly what it was, but to koi enthusiasts, that answer is likely to get you dirty looks—at the very least. Contrary to popular belief, a koi mud pond is not just a freestanding muddy-watered hole in the ground that you can put koi in.

With the development of the hobby (and business) of koi keeping, it has grown way beyond a fortunate confluence of beneficial environmental conditions.

A functioning koi mud pond now is a controlled environment with a host of variables that help to give a koi the best (and as close to natural environment) possible for it to reach its full potential.

The complexity and effort that is required to build and/or maintain a mud pond is the reason why many koi keepers will let their koi “summer” in an accomplished and experienced koi keeper’s (or breeder’s) mud pond.

Not all koi mud ponds are created equal. Individual breeders often will find that they get different results from different ponds. Some will even transport from one to another to get the benefits from ponds in different regions.

And it’s not a fill and go proposition either. Mud ponds must be monitored constantly. It can make for a risky proposition with so much at the whims of nature. Higher than usual temperatures, acid rain, pH spikes, pH crashes. This article “The Mud Pond Myth” explores the potential downsides of a mud pond in Japanese breeders’ koi growing.

What is the “Mud?”

The “mud” that many breeders have in their ponds is actually clay. Montmorillonite clay is the preferred type. It also goes by the handle Bentonite clay (as a result of a large deposit of it being close to Fort Benton, WY). It comes in two different forms—calcium and sodium.  This calcium laden clay is rich in minerals (which vary in type and purity from one deposit to another), one of whose functions is to aid with enzyme production.

Additionally, this clay is laden with nutrients. These nutrients (as well as the micro-organisms that thrive in it), make up part of the koi’s dietary intake and are a large part of the reason for improved health and color in a mud pond.

Evidence suggests that mud pond-raised koi better handle those parasites that are introduced through various vector means (like bird feces).

In addition, the clay is also believed to possess natural detoxifying qualities—which helps to naturally regulate water quality. Through a process known as flocculation, the negatively charged ions in the clay itself, attach to the positively charged elements (like toxins, dissolved organic compounds and bacteria) and create a “clump” that can then be filtered—thereby detoxifying it.

Not all mud ponds have the clay necessary, and so will have to have it added. Some mud ponders use the clay that is

Additionally, not all mud ponds are naturally occurring. Many are man-made.

In man made mud ponds, trying to recreate the water softness can be an expensive prospect. As can seepage.

Water quality

“All water is not the same”

A lot of a mud pond’s “charm” comes from the type of water that flows into it. In Japanese mud ponds, a lot of the water is runoff from mountain snow (similar to the pacific northwest) which is soft water (not as many minerals).

Remember, “dirty” water doesn’t necessarily mean bad water. Nor does “clean” water mean good water. In fact, the opacity and turbidity of a mud pond is what helps to protect a koi’s skin. The mud particles have nutrients and toxin-reduction qualities, and in the summer months it protects the koi from sunburn.

A mud pond is not a viewing pond a la your average backyard pond. It is designed to provide a place for koi to grow and reach their full potential in size, body conformation and color away from prying eyes. It can be quite a treat to see how your koi develop after a season hidden from plain sight.

The murkiness is the suspension of the “mud” particles in the water from the koi’s movements as well as their rooting through the mud (which can be composed of many different types of materials).

There are other factors that can affect the water’s clarity:

  • Spring water or groundwater?
  • Amount of runoff from surrounding area?
  • Land usage in surrounding area (farmland, crops, livestock)?

If your peace of mind involves needing to see your koi in the mud pond, one method of seeing how mud pond koi are coming along is to set up a feeding ring towards the edge of the pond, luring them in for inspection during feeding time.

Soft and hard water

Believe it or not, the hardness or softness of the water koi swim in can directly affect a koi’s colors (which is discussed in this article Koi and Color Loss). The snow melt runoff in Japanese mud ponds resembles distilled water and tends to be softer with less mineral content.

Harder water (or water with a higher mineral concentration) can bring out the sumi in koi, while softer water (lower pH) will create more vibrant reds). With the calcium and minerals in the clay to offset the relatively low calcium levels in the water, the koi can get the best of both for the reds and blacks—deepening both.   

The ponds may be similar but what’s in the water is not! Japanese mud ponds often rely on snowmelt and spring snow runoff to fill the mud ponds. And the water that does run down through the mountains and the soils has different types of minerals, usually less concentrated than the groundwater that is used elsewhere.

When Do Koi Go into a Mud Pond?

Koi are generally introduced into mud ponds in peak growth season, generally April through November. Tosai (one-year-old koi) go in a little later, May, and come out in October.

Two-year-old koi (Nisai) usually spend a little longer in the mud pond, going in in April and coming out in November.

Harvesting a mud pond

What are a Koi Mud Pond’s Advantages?

There is more to a mud pond than the mystical allure of mist-covered waters in the Niigata Prefecture. There are actual benefits.  The main draws of a mud pond are:

Lower stocking levels

In general, mud ponds don’t reach the same stocking levels as commercial (and sometimes koi keepers’) ponds.  With more gallons per koi (which in some instances can be upwards of 100,000g/koi), there is less competition for food or space.

More space + more food + less competition = less stress and healthier koi.

Natural Diet

The conditions for an abundance of naturally occurring foodstuffs are all present. The clay and “good” algae that results from a natural pond is perfect for microbial growth that will feed the types of creatures that koi will eat. Koi are natural foragers and with all of the food at their disposal they can graze at will.

The list of foodstuffs that can make their way into a mud pond is impressive:

Insects, insect larvae, earthworms, various forms of shrimp, aquatic plants, seeds. In fact, koi can get up to almost half their diet in a mud pond from the various naturally occuring fauna and flora.

Beneficial Mineral Deposits

The clay that forms the bottom of a pond will have natural mineral deposits in it which means that the keeper will not have to add it.

Stronger Immune Systems

Evidence suggests that mud pond-raised koi handle parasites better in that environment than they will in a liner pond, as a result of the more natural diet and water conditions.

Temperature Fluctuation

Being in a larger body of water, the koi won’t be subject to large temperature swings, as the large volume of water will take longer to heat or cool. It makes their life just that much more comfortable.

But like anything involving nature, there is never any 100% guarantee of success or a koi reaching its full potential in a mud pond. Most of it is science, but some of it falls into the category of faith.

It is also very important to remember that even mud ponds will need seasonal maintenance. A mud pond, thanks to the more natural processes brought about by the soil depth and composition, will endure longer, but not indefinitely.

A mud pond will still need a freshening up periodically. This can involve draining (usually in the winter), liming (to neutralize parasites, raise pH and calcium content, boost microbial growth, carbon dioxide removal), re-tilling the bottom, regrading and perhaps even fertilizing.

What Makes the Japanese Mud Ponds So Highly Prized?

Mud ponds are not a uniquely Japanese method of enhancing all the best qualities in quality koi fish. They can be created almost everywhere. But Japanese mud ponds do have their own draw. And they are hard to get just right.


You may be able to recreate something, but there is something to be said about those that have done it longest and done it best. The trial and error, the successes and failures, experience and expertise that imbue those koi mud ponds with more than just clay and murky waters.


In Japan, much of the koi breeding has been kept in the family from generation to generation. This is something that is lived and passed down, so along with understanding the procedure, Japanese breeders can recognize, problem-solve and adapt based on years of knowledge and practice.


This is where nishikigoi was born. It is the genesis environment where all of the qualities that make it such a highly prized species come from.

Snow runoff, soft water, low pollution, high elevation, high amounts of rainfall, cool nights, mineral-rich earth (a combination of volcanic and mountain soils), hot and humid day temperatures all combine to make ideal conditions for koi to reach their maximum potential.


Show winners consistently come out of top breeders’ mud ponds. It makes them a top destination for those koi keepers looking to get some of that “magic” for their own koi.

Building a Mud Pond?

Before you put a shovel in the ground (if you prefer the manual approach), perform a soil test. A good start is to check the soil type on the Natural Resources Conservation Service website. The reports will give you some important information on what you are dealing with.

How one man turned a water hole into an all natural pond

You can also chat to some of the folks in your local Soil Conservation Service offices and see if they have any insights as to the viability of creating a pond. Their experience can be invaluable, and they even might have some anecdotal evidence as to how ponds have done in the past.

Once you know the soil consistency, digging down a fair ways (some mud ponders recommend as deep as 12ft) will tell you what you are dealing with.

DIY soil test

  • Roll the soil into a ball
  • drop it on the ground
  • If it disintegrates, there is more than likely not enough clay
  • If it stays a ball, your clay content is probably sufficient

Ideally, your pond’s bottom is lined with heavy and tightly packed clay (at levels of 40% or more). What you are looking for is how the soil’s characteristics will impact water seepage (or how well it will hold water), and the type of ion exchange you can expect. Clay is negatively charged and can help to keep the level of positively charged toxins down.

Basic Building Guidelines

Bigger is better

The larger the body of water, the more stable it will be. The smaller ponds are harder to upkeep.

It needs to breathe.

It might not require a lot of things but it does need to be aerated. Make sure to have aeration available when needed.


3-4’ deep with sloped sides is a good place to start. Shallower, like smaller, is more difficult to maintain.

Get the report

If you are adding your clay, ask for the report of what heavy minerals it contains.


You will have to drain it eventually. Having a means of moving the water out is a must. Additionally, if you do get too much water in your pond, it will need somewhere to go.

Things to prepare for:

Water costs. A natural spring or other groundwater is great, especially if you have seepage or lose water elsewhere. If you are using water from your local municipality, you could be in for a hefty water bill.

Methane gas

Pockets of gas can form in the mud. This can be lethal to your fish. Bigger fish will forage in the mud and help to release some of the gas. Smaller fry will struggle which can lead to dangerous amounts of gas. Some enthusiasts will “walk” it to allow the gas to by bubbled up and out of the mud.


Can’t forget about those looking for a meal. And there are many of them. And they come in many different sizes. Some large (raccoons, otters, blue herons, cats), some not as big (snakes, turtles, frogs) and some quite small (dragonflies and their nymphs). Take precautions to keep them out.

pH fluctuations

Things like acid rain and runoff from around the pond can affect the water’s pH balance. It is still important to monitor the parameters.


There will be some in your pond. A healthy koi can stave off infections and the unwanted advances of these hitchhikers, but they can still become an issue.

A mud pond can be a marvelous place for your koi to summer, but it might not be. You may get maximum growth and color pop. You may not. Some people swear by them, others remain skeptical. But the anticipation (either way) of seeing your koi after a season in growth camp is exhilarating.

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