All fish may start out small, but some don't stay that way. Most koi fish have the potential to grow over a foot long, many over 2 feet and some over 3 feet in length. Unless you have the giant granddaddy of all tanks or need to temporarily house your koi inside for the winter, keep koi out of that tiny fish tank. An outdoor pond is best suited for housing koi long-term.
Minimum Requirements for Indoor Koi System
If you have no choice but to keep your koi inside, you will need to outfit your system to handle their size and waste output. If you're thinking, 'Well, I just won't feed them much, so they won't grow,' would you do the same with your other pets? If you don't have the space to keep koi, it's a good sign that you shouldn't get a koi. There are many other fish species that take up significantly less room that would be a much better choice in a limited space situation. Goldfish can often be found in colors and patterns that resemble koi, but they are only 10-12 inches when they are full grown in most cases. Because of this, they are better suited than koi for indoor aquariums.
A 500-gallon tank is a bare minimum for even two or three koi. In a typical koi pond, you should expect to keep one koi per 250 gallons of water. Large, reproductively active female koi will need 500 gallons each to maintain their egg mass. These more conservative calculations serve pond owners very well by providing better water quality and easier maintenance. Overstocked ponds can have severe issues with poor water chemistry and can cause stress in fish without any room to spread out.
No matter the pond or aquarium size, you will need to have at least three koi. Koi are communal fish and do not do well on their own. Some do OK in groups of two, but a three-koi school is the recommended minimum.
If you are planning on a large indoor aquarium, be sure it has enough room for your koi to turn around easily. A large thin aquarium, such as one built into a wall, will not suit koi, no matter how many gallons it holds. Repeatedly banging into the tank walls or having a narrow space to turn around will result in crooked backs and damaged fins.
Koi, like other carp species, make a lot of waste. Eating heavily and constantly is part of how they're able to grow so large and maintain a massive body size. When there's lots of waste coming out, your filtration better be able to keep up. Again, please don't starve your koi for convenience. Before you decide to keep koi, make sure you can keep up with the maintenance.
If you are starting a new aquarium, beware of New Tank or New Pond Syndrome. In this syndrome, the biological filtration has not yet been established, therefore harmful spikes in ammonia and nitrite will occur in the water before fully cycling to safer nitrate. It may take 4-6 weeks for this process to occur and elevated ammonia and nitrite can be very dangerous to fish. If you are overwintering your koi indoors, be sure to bring some of their filter media along to add to the new filter in order to prevent this. The beneficial bacteria in the old filter media will grow in the new filter faster than if you are starting from scratch with no bacteria in the biofilter.
Net on Top
With limited space and a foreign, indoor environment, koi are more prone to jumping out of their pond. Although the need for predator deterrent indoors is minimal, unless you have a inquisitive cat, you should net the top of your indoor pond at least for the first few months. Once your fish have settled into their new environment, you can slowly move the net off in sections. Monitor your fish to ensure they are not attempting to jump out of the smaller indoor enclosure.
Koi are temperate fish, so they do not require a theater. If you are bringing your koi indoors for the winter in order to keep them from freezing solid outside, you should keep your water temperature at least 39 degrees Fahrenheit /4 degrees Celsius. At temperatures from 39-45 degrees Fahrenheit /4-7 degrees Celsius, your koi will not swim much or be hungry and that is to be expected. As the spring temperatures warm their main pond, try to wait to move them back outside until the water temperature in the pond matches their indoor water temperature.
For permanently indoor aquariums, you should not need to worry too much about temperature. Room temperature water around the high 60s to low 70s degrees Fahrenheit / 18-24 degrees Celsius is great for koi. Expect them to be fairly active and plan on feeding at least once a day. You will not see much fluctuation in temperature throughout the seasons indoors, so you can expect little to no spawning activity, which can be beneficial to many female koi who would rather avoid the violence of spawning males. Spawning is triggered by cooler water temperatures warming to 64 degrees Fahrenheit / 18 degrees Celsius or above in the springtime. Koi may not spawn if the water temperature is above this year around. However, female koi that do not release their eggs over many years can become "egg bound," especially if overfed so that they don't resorb the eggs at the end of spawning season.
Overall, it is not recommended to keep koi indoors. If you are overwintering them, this is a temporary setup and a better alternative than being frozen into a solid block of ice. If you want a permanent indoor setup, space is critical. The bigger the better for koi—and remember they need large water filters.
Be sure your indoor pond or aquarium has room for a fully grown adult koi to turn easily and not run into each other or the walls. You will also need to maintain a robust filtration system. Keeping koi indoors may not be easy, so if you would rather not deal with all that, how about some nice tiny tropical fish instead?