Koi Rescue
Phoenix Arizona

Koi Pond Design Guide

If you wander around in online koi forums and get advice from online experts, they all seem to talk in exact terms and there are many common myths they pass around and warn if you don't follow them, you are doomed.

I have learned from rescuing koi from so many different ponds, there is no single perfect pond. There are many, many different ways to have a koi pond and many different types of systems that work, and the proof is the koi were living there for years and doing just fine.

Below are some suggestions which are based on my personal observations from ponds and systems I have been able to see running while rescuing koi, plus my experience running my own koi tanks (both rescue tanks and breeding show koi for sale).

Why Koi Need a Lot Of Filtration

About five thousand years ago, Goldfish decended from Crucian Carp who which naturally lived in ponds. We know their age, because they appeared in Chinese artwork that was dated. The chinese kept them as a food fish, and the gold ones started appearing. Being a novelty, the Chinese kept the gold ones, and since they could make large ceramic bowls, they started keeping the fancy gold ones in their homes. The goldfish would spawn in the bowls, the ones that could take the poor water conditions survived, and today we have a very hardy fish which can live in horrible conditions. Since keeping goldfish was so easy, they are known as "the people's fish".

Common river carp first appeared in Persia and have been traded across the world as a food fish, and they are the most prevalent fish in the world. The carp have been intentionally introduced into public waterways as a food source, not only in ancient times, but also in the early years of forming the United States, our goverment launched a program to populate our public waterways with carp just as many other countries have done before.

The Koi we have are genetic mutations of river carp. Koi are incapable of producing some of the pigments that river carp do, and so instead of having a nice camoflauge color, they have fancy patterns. Koi have been selectively bread for about 1000 years to enhance the color patterns and also modify various body shapes, such as trying to get the mouth to point more forward, and have specific shapes to the thickness of their body parts like trying to smooth out the big hump in their back.

Because Koi decended from the common river carp, which evolved in a river environment - they have biological systems that prefer lots of fresh water like you will find in rivers. So the filters we use need to be able to clean the water a lot better than what you would use with goldfish or other types of fish.

Pond Size

I suggest a minimum of 1000 gallons for the first koi, then 100 gallons for each additional. This might seem like a lot of water when you look at a little 12" koi, but you have to keep in mind that the koi is going to continue growing and it will be very big in just a couple of years. Serious koi enthusiasts have 1000 gallons of water per fish. Many somewhat-serious koi enthusiasts have ponds in the 3000 to 5000 gallon size range, and the super serious have swimming pools that are converted into ponds in the 15k - 30k gallon size range.

That being said, the vast majority of rescues I have done are from 500 gallon ponds, and the koi had been living there just fine for many years. The reason for rescue was mostly because of the house being sold, new babies in the family, or other koi non-health related reasons.

Pond Depth

The two main predators that attack koi are Herons and Raccoons. They both hunt in similar ways, they wander into the pond in the shallow end, and then wait for a fish to come close, and then strike. One of the best defenses against this is to have a pond with walls that go straight down for 24" or more. As for overall depth, koi like deep water, like in the 6' to 8' range. This is not practical for most casual pond owners, so please consider atleast 24" or greater depth. Many typical ponds I see are 3 to 4 feet deep.

Pond Shape

Because Koi decended from river carp, they love to have a constant current to swim in. A perfectly round pond (a cylinder) is very easy to get a current running, and is the ideal shape for koi. I have round tanks I keep my koi in, and just to keep it interesting, I'll reverse the flow about once every couple months so it goes in the opposite direction. Most pond owners prefer to have other shapes, so do what you can to make a current they can swim in.

How a Biofilter Works

A pond filter works very differently than a pool filter does. What is going on is that koi eat food, then:

The bacteria described above grows on the surface of everything in your pond, the only problem being that the pond walls and pipes in your pond do not have enough surface area to grow the amount of bacteria to process your waste products. So chambers full of "media" is put inline with the circulating pump, and the bacteria grows on the media.

The media could be lava rocks, pea gravel, bio balls, pvc shavings, pot scrubbers, bubble beads, or whatever - the point is to make a large surface area for the bacteria to grow on. As the water is pumped thru the media, the bacteria can feed on the waste products and do their conversions.

Sizing your Filter

As you can see from above, the size of filter you need is based on the amount of waste your fish produce, not the water volume. A good general rule of thumb is that you need 10 gallons of media for each 24" long koi. If you have 5 of 24" koi, then you need 50 gallons of media. Some media is more effecient than others, so you might need more of one and less of another.

For a more detailed and technical explanation of how much media you need, see Koi Pond Filter Media Calculations

Pump & Flow Rate

The minimum actual flow rate you should be atleast half the pond volume, per hour. So if you have a 1000 gallon pond, the actual flow rate of your pump should be atleast 500 gallons per hour. It is much better to have a flow rate which will turn your pond over 3 times per hour. The higher flow rate will help aerate the pond better and make your biofilter run more effeciently.

Note that I say actual flow rate. The rating of your pump is how much water it can push at the exit port of the pump with no load. As soon as you push water through a pipe or above the water level, the load goes up and that reduces the flow rate.

Testing your actual flow rate is easy, just get your filter running and put a 5 gallon bucket under the discharge and time it. Typically pumps will run with at an actual flow rate of half the speed indicated when using the various standard filters I see people using.


There are many types of algae, and it is GOOD for your pond - the koi eat algae, and it helps the biofiltration. The problem is with the free floating "pea green soup" algae, it blocks your view of the fish which you spent all this time and effort to have. The best way I have found to get rid of the green soup algae is to setup a UV sterilizer with it's own pump.

A UV sterilizer is basically a light bulb inside a tube, and as you push the water past the light bulb, the UV light rays kill everything. Each UV sterilizer has a manufacturer's rating of how fast you should run the water through it, so you will want to get a seperate pump that matches that flow rate.

After you use the UV to kill the free floating algae, the algae on the walls will have a better chance at growing and will take over the job of consuming nitrate. Your koi will eat the algae off the walls, and not only is it good eating, but algae has compounds that enhance the red colors in koi.

Typically you will want about 9 watts for around 700 gallons. The bigger the UV the better. The bulbs only last for about a year though, so you will need to get a new bulb each year.

NO Rocks In The Bottom !!

Home aquariums have rocks in the bottom of the tank. These rocks act like filter media, the water is circulated through those rocks and it is a very effective filter, for an aquarium.

In a koi pond, the rocks in the bottom do the opposite, they collect waste and create a toxic muck layer. If you disturb the rocks, they will release those toxins into the water and you can kill all of your fish within a half hour. No kidding, one pond I visited had rocks in the bottom, the owner walked through his pond to clean it up and disturbed the muck, and accidentally killed all his fish.

It is best to have a smooth bottom pond so the muck and all waste will be drawn into the filter system and trapped in there. Then as you backflush your filter, all that muck is removed as it is disturbed so it won't poison your water.

Keep A Backup Tank On Hand

You should have some kind of backup tank on hand to temporarily store your koi incase you have a pond leak or need to do some kind of maintenance. Look in the articles section for ideas. Could be a stock tank, above ground swimming pool or soemthing else. Very important that it be fitted with a net to cover the tank, to prevent the koi from jumping out.

There are a number of other uses for your backup tank. When you adopt or buy new koi, you can keep them in your tank for observation before releasing into your pond. If they have health issues, it is easier to treat in a small tank. If you have koi that you want to adopt out to others, you can put them in your small tank so when the customer comes, they are easy to catch. Another fun thing to do with your small tank is to use it for breeding.