How Many Koi Can I Have in a Pond?

Published on May 22, 2023

This seemingly simple question is actually very complicated. Based on size and design, each ecosystem must have its own distinct fish load, water flow rates, oxygen levels, filtration system, the number of aquatic plants, the types of plants involved, feeding schedules, food types, the frequency of water changes and ratios… and the list goes on.

We generally offer two rules of thumb regarding the number of koi in a pond. First, average water conditions will allow 1 inch of fish per 10 gallons of water (or 100 inches for a 1,000-gallon pond). Second, due to pheromones and toxin control, we want to stay at or under four koi per 1,000 gallons of water to promote a healthy living environment. These recommendations are a starting point and would be the all-encompassing answer to the question if it were a simple question. But since we love our koi, we need to have a deeper consideration for their needs to secure their healthy ecosystem.

Going Big? Decide Early.

This is why one of the first conversations between the owner and professional must focus on the end desire and how many koi the pond owner would like to have. This conversation must happen before you start the design so that the future koi and the pond owner will have the environment they need to maximize their potential — the enjoyment of your work.

Aqua Ultraviolet
I have too many customers with small water gardens that start the fish-load conversation with comments like, “I wish the pond were bigger.” As it turns out, these customers’ expectations of their pond are usually never met, because they want more fish than their pond was built to handle. To truly provide these types of customers with years of pleasure at their pond, you must not limit their numbers of fish by building too small or using inferior equipment systems.

Koi need space to grow, swim, exercise and explore. We not only want to look at pond square footage, but also consider water depth, as this will control the temperature during hot summer months. The total volume is also key, because as we increase the mass of water, we create a safer buffer to disperse toxins and nitrates, minimize rapid temperature changes in the pond and give the koi more space to cruise around and build muscle. I personally do not like seeing koi in ponds smaller than 2,000 gallons, because we really cannot provide both the depth and the horizontal space they need to be healthy when they grow into adults larger than 20 inches.

Position your client for success by building a koi pond rather than a water garden, and use the equipment their koi family members deserve.

This is not to say that you cannot create a very healthy ecosystem for a few koi in a 2,000-gallon pond with proper water flow, filtration and aeration — you absolutely can.

Water Flow & Aeration

Water flow and aeration accomplish the same main goal — keeping the water cool and rich in oxygen. With more oxygen in the water, the ecosystem will be healthier. The fish can swim and breathe more easily, and beneficial bacteria like Nitrosomonas and Nitrobacter can reproduce and convert toxins.

stocked koi ponds
This 7-foot-deep koi pond is not only crystal clear, but also has optimal water stability and health from the oversized bog filter. Size, volume, filtration, aeration and plants create the perfect ecosystem.

By rule of thumb, we like to circulate koi ponds once an hour. However, we can increase oxygen levels and create a healthier ecosystem if we increase the flow or add an aerator to the pond, thereby increasing the capacity of fish the pond can handle safely. Your aerator is a supplement, but it should not be overlooked, especially in deeper ponds that need circulation on the bottom. Air pumps are easy to install quickly and can instantly increase a pond’s fish-load capacity, in turn fostering a very healthy ecosystem.

Your filter choice is the backbone of the koi pond ecosystem. The filter serves as a breeding ground and housing area for beneficial bacteria to do their magic, so the larger or more efficient your filter, the better job the bacteria is going to do with filtering. Oversizing the filter for your pond can permit a higher fish load, because a larger filter system can handle more fish.

Along with size, we also consider function. Filters that backflush frequently are getting cleaned more often and thus are ready for more action. These filters are preferred over filters you clean monthly or quarterly, because biowaste is removed more quickly. Avoid installing filters that are hard to clean, like putting rocks and filter pads inside a waterfall box, for example. Customers need to be able to clean the filter at least monthly, so you have to make it easy for them.

Another piece of filtration that needs to be installed is a properly sized UV clarifier. With the ultraviolet light’s ability to drastically reduce algae reproduction, the filter is inherently more efficient in breaking down fish waste and avoids getting filled with algae.

Nature’s Filter

Yet another filter option is adding plants. A bog filter that incorporates plants and a gravel bed in the filtration process will provide nitrate control and added oxygen through aquatic photosynthesis. The presence of plants is greatly encouraged in all ponds as a supplemental filtering agent and ecosystem protector.

stocked koi pond
Design with the koi in mind, and then create the beauty for your client. The specs, equipment choice and health of the koi must all come first.

Waterlilies provide shade, which cools the water table, while large-root bog plants such as iris, reeds and cannas provide constant toxin
reduction. Possibly most important are oxygenating plants like Anacharis and Cabomba that can stabilize the water table and absorb enormous amounts of nitrates while providing high amounts of oxygen back into the water throughout the day. One bog plant or oxygenating plant per 10 square feet of surface area and up to 50 percent waterlily coverage is ideal. These are easy to add to the pond and will make a huge impact on both the ecosystem and the visual softness, which your client will definitely notice.

The Necessary Extras

Fish load is truly all about water quality, which is good news, because you have control of water quality as you design. The pump, filter and aeration systems are key, but you cannot stop there.

Install skimmers and bottom drains to increase the quality and purity of the water table. If we allow sludge to build on the pond floor, we lose some of that quality. Designing the pond so that all the fish waste exits quickly, gets trapped in the filter and then is released weekly will greatly improve the water quality and thereby increase the fish-load capacity for your customer.

So…Final Answer?

Knowing that koi lovers will always want more koi, it is of greatest importance to build a pond with the proper equipment that will allow the owner to extend the fish load as desired. Ideally, the pond’s maximum fish load should always be greater than the number of fish the client has or wants at the moment.

stocked koi ponds
Our koi are our babies. As social creatures, you can pet them, hand-feed them and even train them to eat from a bottle, so their happiness is very much dependent on us.

In the end, you determine how many koi your client can have with your design and build. You are the professional, and the client needs your expertise in the design as well as your advice regarding maintenance and water quality. You above all others can explain the system’s strengths and limitations. For the pond owner’s future enjoyment, make sure to explain the importance of weekly backflushing and partial water changes. Also help them with food choice and the proper way to stock their pond a little at a time.

So maybe the question doesn’t have an exact answer, but you have control of the answer. Position your client for success by building a koi pond rather than a water garden, and use the equipment their koi family members deserve. They will thank you for years to come for your attention to detail and shared passion.

Shane Stefek is an expert on koi fish and owner of Water Garden Gems  based in Marion, Texas.

Japanese Koi Kodama

23 thoughts on “How Many Koi Can I Have in a Pond?”

  1. The question does have an exact answer. John Russel of Russel’s Water Gardens determined how to arrive at this answer several years ago.

  2. However, when I talk to people (consumers) on the phone, they already have an overstocked pond. It is generally vastly overstocked and I doubt they could have anticipated the “koi-buying-sickness” that strikes new pond owners. I love the article. I think it is a great way to instill the responsibility on the homeowner right up front when installing a new pond as to stocking this pond. Gives them something to think about. Hopefully their conscience will kick in before the fish start dying and maybe send them back to the initial installer for upgrades, if needed.

  3. Here is the link to John Russell’s article (of the same title):
    However, I loved Shane’s “slant” on this in his article in Pond Trade. It is a dialogue that has to start before the pond digging has begun or the hand shake at the beginning of the deal. Put the responsibility of the pond (inhabitants and stocking) on the shoulders, squarely, of the pond owners. Let their conscience be their guide. It will build trust in the relationship with their contractor! Very good article and simply written.

  4. Ron B. - California

    I have a 1800-2000 gallone pond where we HAD eight nice looking koi. Turns out that things may have gotten a little out of control as I was late in discussing family planning with the group. We now have eight PLUS 3 newly discovered “babies”. What do you suggest I do? They obviously did not listen to me about proper birth control!

  5. Ron, This is not an uncommon occurrence and you have several options. If you want to keep all 11 fish, I would recommend adding aeration and go to a routine 10%/week water change or something to that effect to help control the nitrates and water quality issues that may arise from overstocking. I would also look at your filtration systems and if it is one of the high quality Koi Filters, you probably will be alright with your family size…but if it is not, I would recommend taking the opportunity to invest for the future health of your pond and upgrade. You can feel free to call us at Water Garden Gems for more specific guidance or another expert Koi specialty location if you would like. If you are looking to share some of your family, check into local Koi Clubs or Koi Rescue services and see about finding a new great home for some. Again, if you want to detail out your system beyond the raw number of fish, give us a call and we are happy to help you.

  6. I have a pond that calculators would suggest is 2100 gallons. It runs between 24” and 36” deep.
    Currently I have six Koi in this pond ranging between 16” and 22” with the total being 120 inches.
    I am running an Aquaforce 2700 pump and a picked up a Laguna MaxFlo 950. I have two Laguna Pressure Filters – (1400 and a 2100 models). The bottom is granite river rock and granite bolders.

    I am planning to continue with a waterfall (3’ high and about 7’ long) in the spring that will split into two streams spilling into the 24” deep pools at the shallow end of the pond.

    I have installed 6X6 columns for a pergola at the perimeter of the pond. The cedar beams crossing over top of the pond support a monorail allowing me to run a 9’ X14’ ShadeFX canopy over the water. So I have the ability to provide considerable summer shade and sky-view-protection from Heron’s in the summer. My pond has other shade elements (a large granite slab overhangs the water in the deep end and there is a 3’ diameter mill stone fountain in the deep end, as well.

    Even after head losses for the waterfalls, I suspect I will be capable of circulating 1 ½ times my pond volume every hour.

    Other things I could bring into the equation:
    I have two aerators, if required
    I have a Pondovac 3 for occasional maintenance of the bottom

    My Questions:
    Full Time Residents
    I would like to add two more larger 20” Koi (Sanke and Asagi) to my pond – increasing the fish length to 160 inches.

    Temporary Guests
    I also have the idea that I would like to buy small (6”?) high-quality koi in the spring; allow them to grow over the summer; and sell them off (not necessarily for profit) in the fall. If I were to attempt this, how many of these smaller fish could I include, given the compensating factors I have talked about above?

    1. Doug,
      Thanks for your question. I would first say YES to using the aeration you have available. The more oxygen the better. Adding a couple nice Koi to the mix should be perfectly fine, but I would recommend frequent backflushes of your pressure filters as you will at that point be maxing out your system so I would expect the filters to fill up rather quickly. I personally would probably backflush them weekly, of course paying attention to the amount of waste you are removing and adjust accordingly. I would also make sure that you have good circulation along the bottom of your pond to minimize the waste settling into the pond floor and getting stuck in your aggregate. Definitely vacuum the pond…at least twice a year…at your major season changes. Food choice is important as well and you will want to feed a high quality food that leaves minimal waste such as HaiFeng or some of the other well respected Koi Food Brands.
      As for your temporary guests, I think with due diligence on your part keeping the ecosystem clean, there would not be a problem. Keep in mind though that some of the key elements required in “grow-out” situations such as you are describing are water quality, O2 levels, and feeding (a lot of feeding…2-3 times a day) and so a pond that is already near it’s fish load is not your optimal environment. My only other recommendation here is know that your pond is going to be full…so you will not want any of those temporary guests to change their status to permanent citizens.

  7. I am building a round concrete pond 7 feet deep half set in earth half above and walled around. It will have an 18 foot diameter and hold some 12,834 gallons of water turning over 7000 units of water an hour from pumps. I am working on 40 fish but can be corrected by expert as this is my first big leap from a small pond to something better in my roomed gardens

  8. Kevin,
    The answer would be found in your filtration system, aeration levels, and water temperature and then in sub components of your pond such as feeding habits, pond floor circulation/cleanliness (bottom drains or jets), etc.
    You definitely do not have too many fish for your volume of water, but aeration levels would be something of a possible concern with only circulating the pond once every two hours. If you are in a cold climate, I would not be as concerned for the health of the fish and the water ecosystem, but if you are in a warm climate as I am in San Antonio, Texas you definitely need to rotate your pond closer to once an hour and use aeration systems to push oxygen down to the bottom of your 7′ deep pond and keep the water cooler.
    With a great filter system sized properly like an Ultima2 20,000gallon filter and a Matala 150watt UV unit, your water quality should be great for your 40 Koi without issue. At my personal pond at home, I have this systems with 40 Koi in a much larger volume pond and have crystal clear water year around and can count the pebbles on the floor of my pond 7′ deep. I do though circulate my water hourly and have a Hakko 60LPM aeration system to support the ecosystem.
    I would watch your nitrate levels periodically and make sure you are doing routine water changes to flush some of the toxins and hormone releases out of the pond…we recommend 10%/month water change and if at all possible, to be pulled from the floor of the pond or through a vacuuming process to help pull the worst things out.

    Thanks for your comments and we hope you love having a LARGE Koi Pond…

  9. Thanks for the response.

    I think I will take a two-year approach to this. Over the winter, I will come up with a plan for pumping and filtration and look at flow in the pond. In the spring I will do a thorough cleaning of the pond with the vacuum and add in the skimmer box. I will continue running aeration through the summer. I managed to pick up a 28W Laguna UV Clarifier and will put this in-line with the system. I will build the back-flushes into my weekend routine. I have clear-line hoses for back-flushing.

    I will add the permanent guests in the spring and delay the idea of temporary guests a year.

    Thanks again

  10. I have been doing aquarium fish (discus, angels, severums) for over 30 years. Once we moved to Florida, I wanted to expand my fish hobby outside. 2 years ago, I built a water garden of about 350 gallons in the corner of my back yard. I had a few Koi (maybe 3 or 4) and the rest were shabunkins, about 12 fish in all. Like most, I wanted a bigger space, so I dug out a place for a 3000 gallon pond in my patio area. I had no problems with my 350 gallon pond other than cleaning the filters. I intend to move these same fish to the larger pond next weekend and add NO other fish. I will use the same pumps (Aquascape 2000) and filter (generic 4000G filter with 13W UV clarifier), which are already functioning perfectly, and added 2 additional pressure filters (Danner Pondmaster Clearguard 5500 with 18W UV and (SDS Pond pressure Bio Filter 4000GAL with 13W Sterilizer light) and 2 additional pumps (Aquascape 91012 and Aquascape Aquaforce 5200). So I am running 3 filters and 3 pumps on a 3000 gallon pond. I am in the military and frequently away from home, so maybe you can understand my desire for LOW maintenance. Three filters and pumps on one pond is definitely overkill, but my maintenance and back flushes are quicker and less frequent. Does anyone have any cautions about what I am doing? Additionally have an EasyPro quick sink self-weighted pond air diffusor, an Airmax Koi Air water garden aeration air kit and two Aquascape waterfall spillways, so I should have plenty of circulation and aeration. Any other suggestions?

  11. Dwayne, Thanks for the questions and we always like to overfilter! There is no problem with running three pumps/filter systems on your pond…I have three on my personal Koi pond. Your filters cover plenty of water/fish load and your cumulative UV wattage is more than ample. Your pump selection sounds like it is paired fine with the filters, but make sure you are not maxing out the filter flow rates as you definitely do not NEED all the flow you have. Aeration should be fine as well with two waterfalls running and an air system. I do not know the size of your aeration system, but with the circulation you have built in, it will serve more to move the water table and be a safety net than to be adding oxygen to the water.
    Your fish load is below your pond’s capacity and is very healthy. Unless your current fish decide to multiply, you should have a fairly easy time maintaining beautiful water and very healthy fish! Bravo for investing above and beyond!

  12. Currently, I have a 425 gallon above ground pond with a Lifeguard Double All-in-One Pond Filtering System and the Aquascape 75000 Pond Air 2 Aeration kit. My pond is home to 3 Koi and 1 Pleco at the moment. My concern is that I am overcrowding the pond. Naturally, I want to add more Koi but I want to ensure they have enough space and resources. I do not have any plants yet and I am trying to test water quality as often as possible. pH and Nitrite levels have been fine but I’ve been seeing high levels of Ammonia and Nitrate. I recently did 2 water changes a few days apart to try and lower both. I also used Ammo Lock and Stress Coat afterward. Any additional advice is greatly appreciated.

  13. Jessica, thank you for your question and care for your fish. The ammonia is defiantly an issue you want to remedy ASAP. Water changes are helpful and you may find you need to use an ammonia blocker such as ACCR made by Fritz to neutralize the ammonia burn for 2-3 weeks while the balance in your pond is unstable. If the numbers do not leave off, you may already have too many fish for your filter system and need to make a change on fish load or size of pond. The nitrates being high requires more water changes. This too could be showing that you already have too many fish, but you can add plants and see if they can help balance the nitrates for you.
    As for your fish addition, you really do not have room to add. Once your koi are 10” or so you have reached your healthy fish load max and short of running a flow through water system, you are capped out. The high ammonia and nitrates lend us to believe you are already there. I would strongly suggest enlarging the pond so that your love of your koi and the hobby can grow as you are wishing for.

  14. Hi, I have a 5000-5500litre (1500 gallons) pond that has a depth of 3feet to a meter I would like to know how many koi fully grown you would recommend I have a gravity box fed filter wich is rated 9000-24000 with an 8000 pump

  15. Dani, thank you for the question. The simple answer is six koi once they are 20”+. If you have an extra aeration system or a UV unit that is at least 20watts, you could add and still be safe. Frequent water changes…10-20% every 2-3 weeks will also help keep the toxins like nitrates and hormone levels low in the water. Regardless of your system, short of a semi-constant flow through, I would not recommend crossing 10 total or 180”s.

    I hope this helps. Enjoy!!!

  16. So I’m renovating my garden and I went and bought a pre-formed pond 750l or around 160 gallons. I was hoping to get some nice fish in here so I bought an Oase 3000L pump filter system with a UV lamp built in to it and picked up a waterfall, I’m getting very mixed reviews of what fish I can keep in it. First guy in a fish shop said 3 Koi then the next shop said 4 koi and the next said 0, could I keep 1? If I can’t have any what other types of fish can I keep ? I don’t particularly want normal orange gold fish and what fish can I mix with each other ?

    Sorry for the lack of knowledge , newbie here.

    Thanks 😊

    1. Pez, Koi are not going to be a good option for you due to the size that they will grow. People that say you can have Koi in that size pond are not thinking about what happens in two years when those Koi are 15″ or larger. Not knowing your climate, I will answer with the obvious however you may be able to have other more tropical fish if your winter temperature allows for it. Goldfish come on many varieties and you can get very beautiful Sarassa Comets and Shubunkin Comets as well as Calico Fantails and Sarassa Wakins that would all acclimate in your pond nicely. These fish can handle water down to 40 degrees so they are quite versatile. There are also varieties of fish like the Paradise Fish Gourami that may be nice additions as well for you. These smaller, but beautiful options are more likely to be healthy and happy in your pond and will still give you a variety of colors and tail options.

  17. We replaced our front yard’s maturely landscaped dry stream and dry pond feature with a waterfall and pond after consulting several area builders. Our local Koi dealer suggested a 5’deep concrete pond that looked like a bathtub; instead, we chose a contractor who has experience with natural-looking ponds and are very pleased with the appearance. It’s built with pond liner, 4.5′ deep, roughly triangular-shaped, 11′ x 12′ x 8′, has a bio-filter, skimmer and aerators; the pump capacity is said to be up to 4000 gallons/h, but that would be a torrent; we’re running it on a lower, more appropriate flow for the scale of the installation. The stream bed is gravel lined and the pond bed is river rock. The builder says a bottom drain isn’t needed and suggested Japanese snails. The Koi dealer says it will be great for viewing but will kill any Koi that we get. I don’t know if he’s feeling bad about not getting the construction job, or if our builder is completely over his head on this project. I’m wondering if I should just get some inexpensive goldfish for a while, but we waited till our kids were grown and really wanted Koi.

    1. Avatar photo
      Lora Lee Gelles

      From Carolyn Weise, Ecological Laboratories:
      The little koi will grow to approximately 36” in ideal conditions and you should figure 10-gallons per inch of fish before you add the first fish. I also use 8.5 gal/inch of fish for any type of goldfish. That is based on having good circulation and filtration, of course.

  18. I have ordered 10 7-10 inch koi but my pond is not ready. I want to put them in a holding tank until I finish the pond that they will be in permanently. Given that there will be a pump filter that will circulate the water what size tank would you recommend for a week or so? Thank you. My planning and execution did not sync!!

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