Most koi enthusiasts would more than likely insist that the best environment for a koi to reach its full potential is in an outdoor pond with ample space, more than adequate filtration and more than a few friends to share it with.

However, that might not be a possibility for some koi kichi. Not everyone has the space or the facilities to be able to dig out and build a 50,000-gallon paradise for their cold-blooded living jewels. Living in an apartment is not conducive to a koi pond.

Of course, there are also the aesthetics of koi keeping to consider.

The preferred method to view koi is from above, which is how they are judged at shows. Why? Well, as one koi enthusiast succinctly puts it: “as far as I know, it’s strictly because they were bred for the pattern to be viewed from the top. If you look at them through a window in a pond, they are ugly. LOL.”

While we might not go as far as to say the pectoral view of koi is “ugly,” the dorsal view is definitely more aesthetically pleasing. Although koi in an aquarium can be looked at from above, most aquariums are set up so that fish are viewed from the side.

Nevertheless, some kichi may have no choice but to keep their koi in an aquarium, so we’ve gathered some helpful tips to consider.

Koi Tank Size

Antique cast iron victorian aquarium
Courtesy: ℜ𝔬𝔰𝔦𝔢 𝔚𝔥𝔦𝔱𝔢 @rosalindmwhite

One of the first questions that a budding koi enthusiast often asks is how big their aquarium should be. Koi require a large space to grow into and enjoy. With this in mind, even for a single koi (and, as social creatures, they are better in pairs or groups), you will want a large aquarium to host your koi. When in doubt, always err on the side of bigger being better.

Koi are big fish. They may not be when you first buy them, but they can get upwards of 36” long. Of course, that type of maximum growth requires the perfect storm of environmental factors, but koi will still get in excess of 2 feet in length given a healthy environment. So when you buy a koi (or two) plan your aquarium for the koi you’ll have one year from now, not the little one you pick up today.


Koi grow. Quickly. Given an ideal environment (proper diet, healthy water, good filtration) a healthy koi can get quite big—and very fast. This means that you need to take into account how big your koi can get instead of the size it is when you buy it. “Older “ varieties of koi (like Chagoi) can grow to over three feet in length. It is important to consider the growth rate of koi when purchasing your aquarium.

There are quite a few different theories and philosophies when it comes to determining the size of your aquarium or tank. The easy answer is to use the generally accepted rule of thumb that you will need 10 gallons of water for every inch of fish. Another way of looking at it is 1 cubic foot (width x length x height) per inch of koi.

The calculations for how many fish you should have for the size of aquarium you want might vary slightly from expert to expert, so to make it easy, here is what Koi Acres recommends. This chart is the gallons/koi recommendation per fish in each of the size ranges.

Small Koi 2″ – 8″ 100 – 150 gallons
Medium Koi 8″ – 14″ 250 – 300 gallons
Large Koi 14″ – 24″ 400 – 500 gallons
Jumbo Koi 24″ – 36″ 750 – 900 gallons

There are other factors to consider, too, all of which will have a bearing on the size of the aquarium you will need to comfortably house your koi:

  • How many fish did you plan on having? The more koi, the more space and bigger tank you will need.
  • Are you going to put features in the aquarium? If so, remember that takes up space.
  • Sharp, jagged or rough fixtures can damage your koi’s skin and scales if not given enough space.
  • Are you going to add plants?

Some General Aquarium Tips for Your Koi

Stand size: if your aquarium is sitting on a base, make sure it will be sturdy enough to carry the weight of the tank and all of its contents.

Max bearing weight: be advised that the floor or surface under your tank might have a weight restriction. Make sure that you account for the weight.

Keep it covered: koi can and will jump, especially when stressed. Keep a cover over your aquarium.

Add substrate: many koi enthusiasts warn against using loose gravel at the bottom of a pond, but koi are foragers so they will enjoy sifting through the material looking for food. Make sure it is smooth though.

Keep out of direct sunlight: if you are heating your aquarium, don’t put it where it has the potential to overheat. Make sure you have some shade for your koi if your tank is set up near a window.

Other pets: feline friends don’t just go fishing in cartoons, and dogs can get a little exuberant. Consider how your koi will be received by some of your other family members.

Koi Aquarium Temperature

Koi aquarium built into apartment stepsKoi, as we stated before, are very tough fish capable of surviving in extreme temperatures. That being said, the optimal temperature for koi is anywhere from the mid-60s to the mid-70s (ºF).

This temperature has numerous benefits: it’s good for koi growth, it limits the virulence of Aeromonas bacteria and won’t exacerbate any existing issues, and water at this temperature holds dissolved oxygen better.

As part of maintaining optimal temperatures, try to place the aquarium out of direct sunlight. Same as a pond, superheating in a small body of water is not good for koi. Some shade is necessary for a koi if your tank is set up near a window, as it will heat rapidly. The same goes for drafty or heavily air-conditioned areas.

Try to minimize the potential for rapid temperature changes. Koi can handle temperature variations but rapid changes or swings will stress them out.

Filtration in a Koi Aquarium

Koi are messy creatures. They can produce an incredible amount of waste. In fact, they can excrete up to ⅓ of their body weight in a single day. The importance of proper filtration cannot be overstated no matter where you keep your koi.

In terms of what your filter can and should be able to handle, this Russell Water Gardens article does a great job breaking down the capabilities of biological filters to handle the nitrogenous waste.

Some of the finer points of filtration include:

  • Nitrifying bacteria process weight according to its mass across the filtration system’s Specific Surface Area (SSA)
  • Knowing your SSA is very important
  • Koi produce waste at a 1:3 ratio of waste to body mass.

The long and the short of it is that you will need to do some homework before filling your aquarium with koi if you want them to be comfortable and healthy.

Biological filtration is necessary, but you will also need mechanical filtration to help remove the solid waste that your koi produce or that is left over from uneaten foodstuffs. Additionally, aquarium filters can have chemical components to them that help with waste management.

It is important that the filtration system you choose is able to keep up with waste production and can cycle the entire volume of the aquarium 3 to 4 times an hour.

This article from The Spruce Pets offers insight into the different styles of filters available for aquariums, their general costs and how they operate.

Filtration is a vital part of keeping your koi hale and hearty, and so looking at the best filters is important before buying your koi and aquarium.

Adding Koi To Your Aquarium

koi being introduced to new home in floating bagGoing from one aqueous environment to another can be extremely stressful for a koi. Not only will the water parameters be different, but the temperature is different, too. Making the transition as smooth as possible will go a long way to helping your koi acclimate quicker and not succumb to opportunistic pathogens that may have found their way into your tank water.

The protocol for moving your koi to its new aquarium home is similar to moving them into a pond. If your koi has been shipped to you in a plastic bag, make sure to float the bag on the tank water’s surface first. This allows the koi to adjust to the temperature.

DO NOT dump the water in the bag into the aquarium. It can create a large swing in parameters for the koi already in the tank.

Cycle the aquarium first too. Clear water does not mean clean water. If you haven’t seeded your filter with beneficial bacteria, you can have lethal levels of ammonia or other nitrogenous waste in your tank—and this can quickly kill your koi.

Water Changes

Most aquariums are not going to be as large as outdoor ponds (there are some exceptions, of course), so changes won’t be as megalithic. But water changes still need to happen. Every week.

Water, whether in a tank or an aquarium, is still water. And it still needs to be changed to carry out those impurities that filtration cannot fully eradicate. Nitrates, phosphates and hard waste all need to be removed to give your koi the healthiest water environment. Water changes will also remove those substances that algae use to flourish.

If you have opted for substrate, vacuuming it to clear out accumulated waste products is also extremely helpful.

It also helps to replenish those trace elements used by the koi.

The percentage of water change needed per week will depend on the fish load in the aquarium. The more heavily stocked the aquarium is, the higher the percentage of water will need to be changed. Anywhere from 10-20% is normal every week. If it becomes apparent that there is a bacterial outbreak or some other disease, or if the tank has been treated, that percentage can be bumped up.

This Tropical Fish Success article has some helpful tips on how to change aquarium water.

Oxygenating your Koi Aquarium

Adding dissolved oxygen to your aquarium water is an important aspect of gaseous exchange, and as such, keeping your koi healthy. It is no different than a koi pond.

Simple surface agitation can provide a sufficient 5-7 ppm of oxygen. Often times, the filter will provide this service for you. You can augment it with a submerged air stone or a power head over the surface of the tank.

Plants can also help increase the dissolved oxygen in the water through their own natural processes, too. Your koi might also snack on these plants since they are foragers.

Also, remember to add Dechlor to your water after a change to neutralize any chlorine or chloramine that has been added.


DO NOT, and we repeat, DO NOT simply dump unwanted koi into a nearby waterway. They are a hardy and prolific species. Once they get into an open waterway, they can and will take over that body of water. Look for your local koi club, and they can help you rehome your unwanted koi, giving them a good home and keeping the local waterways (and more, importantly, local government) happy.

You don’t need to have a pond in your backyard to enjoy a living jewel. An aquarium filled with koi is a nice way to brighten up a space. It will require some diligence and substantial space, but keeping koi in an aquarium can be a fulfilling endeavor.

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