Koi fish are known for being colorful, stunning, and massive. They’re the gorgeous gentle giants of ponds around the world and are absolutely beautiful in a wide range of aquascape features.
That being said, there is much controversy surrounding keeping koi in smaller enclosures like tanks and mini-ponds.
Let’s explore what makes koi unsuitable for most common tank enclosures.
Why Tanks Are Not Suitable
Koi fish are gorgeous, hardy animals that can survive a lot. That being said, your goal should surpass survival and extend into creating an environment in which your koi can thrive.
Koi fish are massive, reaching between one and three feet in length at maturity depending on the subspecies you have selected.
This means that those tiny little koi you get from the pet store will be absolutely monstrous within just a few years. Needless to say, they need a lot of space.
Koi fish are so large that it is recommended you house them in a one-thousand-gallon enclosure.
For reference, this usually works out to a pond that is more than ten feet in length, more than five feet in width, and around three feet deep.
This takes up a lot of space and is incredibly hard to maintain in an inside space since the volume of water when held in a massive tank requires frequent quality checks and general maintenance.
This is why outside of professional aquariums, you do not see such enclosures often.
A tank set up like this would also be incredibly expensive. A simple, common one hundred gallon tank costs more than three hundred dollars on average.
This cost does not factor in the cost of filtration, food, or any other care aspect, including something to make the water flow to increase oxygenation since koi do not do well in stagnant, still water.
For most hobbyists, it is simply not feasible to expect yourself to be able to create conditions in a tank that can allow koi fish to thrive on a longterm basis.
Why Do Pet Stores Keep Koi in Tanks?
This is a bit of a controversial topic.
Generally, pet stores keep baby koi in tanks due to space constrictions. They cannot just install a pond due to their space likely being rented or just not having enough room and time to properly maintain it in a commercial setting.
While it is not ideal, as long as the koi sell quickly and do not stay into the tank into maturity, it is generally regarded as acceptable, especially since most breeders use tanks or small ponds for koi fry and peanuts. The issue arises when adult koi are kept in such conditions.
Can You Keep Just One Koi?
Many people who still really want a koi in a tank will opt to just have one single koi.
While not ideal, this is slightly better than cramming several into a too-small enclosure.
Still, a pond is significantly better for their health. Also, a single koi fish will have a significantly shorter lifespan than koi kept with companions.
These gentle giants are very social animals and need the regular interaction to help prevent stress and other health issues.
What About Quarantine Tanks?
Quarantine tanks are one of the few times where using a tank for adult koi fish is considered acceptable. This is due to the fact that you do not keep koi in a quarantine tank on a longterm basis.
Typically, quarantine tanks are just for new koi who need to be checked out before being introduced into the general population to ensure there are no diseases or pathogens that can be spread between the new koi and the old ones.
Additionally, they can be used for sick koi fish or injured fish that need medical treatment that could damage the entire pond if done in its usual home and for breeding purposes to hold the new koi fry and prevent the adults in the pond from eating the koi eggs.
Do Fish Grow to the Size of their Tank?
This is a common myth used to explain why it is okay to house koi fish, goldfish, and other fish species in enclosures that are typically much too small for their species specifications.
While there are many different things that can slow or outright prevent the growth of a fish, including food and water quality, tank size cannot.
Your fish has no concept of how big the tank is aside from it being small causes discomfort; its body does not adjust to the size of the tank. Your fish will continue to grow for the duration of its shortened lifespan if kept in conditions that are not suitable and optimized for its specific requirements.
Do Quarantine Tanks Need Filtration?
Ideally, your quarantine tanks will be set up similarly to your actual pond to help acclimate your fish to the conditions they are going into or make them feel more comfortable while being treated for a given illness, injury, or condition. This includes one of the most important aspects of koi fish care, filtration.
Kois are big and eat a lot, meaning their water may get quite dirty even with the best low-waste, easy to digest food products on the market.
Having a good filtration system is absolutely key in keeping your koi in a clean enclosure where they can continue to grow and thrive.
While you can cut down on decor and hides in a quarantine enclosure, it is best to try and always have filtration and oxygenation to help keep your koi fish safe for the duration of their stay until they can get released back into the pond.
The Bigger, The Better
In general, the rule of thumb for koi fish is that you should shoot to provide an enclosure that is as large as you can.
They can get really big so it is best to assume you are going to have a massive fish on your hands and be cautious with your planning. Once you have a big enclosure ready, you’re all set to bring home some of these amazing fish!