Koi Fish Tanks: 7 Common Questions Answered

Koi can grow up to 36” long and live for more than fifty years in the wild. For those keeping koi in an aquarium, you’ll find that the fish don’t grow as large or live as long. However, you want to give them enough space or they’ll become stressed or even die.

We’ve compiled a list of the most common questions people have about keeping koi in fish tanks. We’ll take the time to answer each question in detail.


Koi Fish

What Size Tank Do Koi Need?

You’re here because you’re asking about koi fish tank size. We’ll start here and say you shouldn’t put a koi in that goldfish bowl on your desk. Frankly, those things are so small that they explain why goldfish in those bowls die ten times faster than their wild kin.

The size of tank koi need depends on their size and how many fish you have. They need, at a minimum, plenty of space to swim around without running into other fish.

How Big of a Tank Does a Koi Fish Need?

Mature koi, the 36” long fish that we often see in koi ponds, need at least 50 gallons apiece. This means that 4 koi that large can only be kept in a 200 gallon tank, and that’s the minimum.

If you want the fish to be healthy and able to grow, then a 12” koi should be kept in a 100 gallon tank.

Younger koi need at least 29 gallons of water to swim in. Conversely, 90 two inch koi could share that same 100 gallon tank, at least until they grow.

What is the Koi Fish Minimum Tank Size?

Ten gallons should be considered the minimum tank size for smaller koi. For larger koi, a hundred gallon tank is necessary for the largest koi to be comfortable.

Note that a tank full of coral, rocks, decorations and other items gives koi lots of places to hide but takes away from the space they have to swim. For example, if your tank is a third coral and rock, you’ll need 33% more water to make sure the fish have plenty of room.

How Many Gallons of Water Does a Koi Fish Need?

How many gallons does a koi fish need? The answer is that it depends on the size of the fish. The larger the fish, the more space it needs. Another factor to consider is the population density of the tank. If you have a 100 gallon tank, one mid-sized koi could be happy if there are a few other small fish but may struggle if there are other large fish in the tank.

A good rule of thumb for determining the minimum acceptable koi fish tank size is one cubic foot of water per inch of koi fish length … per fish. You can determine the fish tank’s height by the width and length to determine its cubic footage. Recognize how far you’ll fill it up, since we don’t always fill the fish tank to the brim.

If you generally fill up the tank 90% of the way up, multiply the water height with the width and depth. This tells you how much water the fish have available to them.

The next step is determining how many fish you have and how long they are. Suppose you have a 4” koi and a 6” koi. That’s 10 inches altogether. You need one cubic foot of water per inch of fish length. Ten inches of fish equals ten gallons of water at a minimum.

Of course, you can put them in a twenty gallon tank. If there are fish other than koi in the tank, then you’ll need more water as well. The only critters that don’t affect this equation are crabs, snails and other invertebrates.

Remember that the tank’s ability to support fish is not just dependent on the water volume but the water quality as well. If you’re adding a bunch of small koi to a tank, make sure that you increase the water filtration and oxygenation to keep them all healthy. The filtration system should process at least 350 gallons per hour for each 12-inch fish.

Sometimes you know how large the tank is in inches but not how many gallons it can hold. A cubic foot holds roughly 7.5 gallons of water. This means a 10 cubic foot tank can hold 75 gallons of water if filled to the brim. Given our prior estimates, you could have 50” worth of fish in a tank that size. Whether that’s 5 10” fish or 10 5” fish depends on your situation.

Can Koi Fish Live in a 10 Gallon Tank?

Very young koi could be kept in a 10 gallon tank. Hatchlings fall in this category. In theory, two 2-inch koi could share a ten gallon tank. One 6-inch long koi could live in a ten gallon tank. Fish any larger than this need to be in a much larger tank.

How Many Koi in a 10 Gallon Tank?

A six inch long koi could be kept in a ten gallon tank, but it may not grow larger or will become stressed if it grows. Several smaller koi could share a tank this size. If the koi in your fish pond had babies, you could throw two dozen koi in this fish tank until you find a home for them.

How Many Gallons of Water Does a Koi Fish Need?

There’s a reason you see koi in fish ponds instead of the average fish tank. Yes, they are easy to take care of. However, because they can grow so large means they will likely outgrow your tank (or die, if in a tank too small for them).

To maximize the health of the fish, each one needs several gallons of water. The larger the fish, the more space it needs. If you have 3 foot / one meter long koi, congratulations on taking good care of them.

But if you need to bring them inside to keep them safe during the winter, each one of those prized koi needs 72 gallons minimum. If you have several such large koi, then you might be looking at a thousand gallon tank.

In a situation like this, putting heaters outside near the pond might be better than trying to relocate them to a fish tank. Increasing the depth of the pond might let the koi spend the winter in their pond.

If that’s not an option, then breaking out 300 gallon storage tanks and turning them into koi fish tanks could give your koi the space they need.