Koi Fish swimming in a tank

Koi fish thrive in outdoor conditions, making koi ponds the preferred area; however, for some enthusiasts an outdoor koi pond is impractical or impossible. There are also those koi lovers that prefer being able to observe their living jewels’ personalities at eye level through glass.

Koi are a particularly hardy fish and can adapt to living in many environments including tanks, aquariums and indoor ponds. Much like outdoor koi ponds, indoor koi tanks have some special considerations to take into account.

Optimal Number of Koi

A major consideration is the number of fish you should keep inside. Overstocking is not good for your koi or their indoor environment.

Indoor options are generally smaller than outdoor alternatives, so the fish channel recommends aiming at a ratio of 100 gallons of water per 12-inch fish.

It is also important to consider what size the fish can potentially reach. The three 6-inch koi you bought two years ago have a tendency to get bigger very quickly, which means you’ll need a bigger tank or fewer koi in the one you have.

Maintaining water quality

In much the same way as an outdoor pond’s water quality is key to healthy koi, your indoor koi pond or tank needs to have the basic elements to keep the water’s quality up to snuff for your koi fish to enjoy a healthy life.

Conditioning the tank (dechlorinating and removing heavy metals) is recommended before introducing your koi, as they are sensitive to these elements.

Invest in a good pump to ensure proper aeration and as well as an adequate dissolved oxygen level.

Water changes are essential, too. Change up to 20% of the tank’s water every 2-3 weeks to help with water purity. This water will also need to be treated before being added to the tank to minimize the stress on the koi.

This brings us to the next essential point.


The importance of filtration cannot be stressed enough. Koi generate a lot of waste products, which quickly affects the water quality as well as visibility. Biological, mechanical and chemical filtration is critical to creating the right environment for your koi.

An external mechanical filter (with activated carbon for chemical filtration) will help to remove those dissolved biological wastes, while a bottom filter (under your choice material on the tank’s bottom) will remove the majority of other waste. Be prepared to clean it weekly.

Other things to Consider

Tank’s location: If using a stand or a base for your tank, it will have to be able to sustain the tank at its full capacity—as will the flooring beneath your tank. Well-lit areas, but not in direct sunlight, are also good for the koi. And try to make sure that if you have other pets that they cannot get to your koi.

Coverage: koi fish are expert leapers, so some sort of cover is necessary. Either netting or a glass top will work

Temperature: koi can withstand a wide range of temperature variation, but adding a heater can regulate it to the preferred 68-77F range. Both your koi and their metabolisms will thank you.

Extras: koi fish are natural sifters, so a gravel or rock substrate of varying sizes two inches deep (minimum) over your filter is a nice addition for them. Beware live plants. Koi love to nibble on them and they won’t last long. Artificial might be a better option.

To help you stock your pond or tank, Next Day Koi offers a wide selection of koi fish that are sourced from some of the best farms around the world. Coupled with a great selection of both sizes and types, we leverage our high volume of shipping through UPS to bring you some of the most competitive Next Day Air shipping rates in the industry.

Contact one of our representatives to see how we can help you stock your pond.

8 responses

  1. Can I bring my koi in from outside for the winter and put them in my community Fishtank?

    1. It will depend on several factors, including the size of your tank, the size of your koi and the filtration that you have available. But yes, it is possible that you can bring them inside for the winter.

  2. No! Don’t put them with your other fish as they produce ammonia which kills all other fish except goldfish ( which are similar to koi ). I tried that this year and replaced 7 fish before I figured it out.

    1. All fish (not just koi and goldfish) produce ammonia. Ammonia is toxic to all fish (including goldfish). If you have proper filtration on your system then the ammonia will not be a problem.

  3. Definitely not true I have successfully established a community tank of koi, angel fish, rainbows and plecos. I run a fluval 306 canister filter and have had zero issues.

    1. Hey Cody, koi can live comfortably with goldfish provided both have room to coexist, and a good amount of food. You will just need to check the length of all fish and compare it to the total number of gallons in the pond. The industry standard is 1 inch of fish per 10 gallons of water. This will tell you how many koi and goldfish the pond can support without overstocking. Thank you!

Leave a Reply

Your email address will not be published. Required fields are marked *