As ornamental fish, koi carp have stunning visual features that would undoubtedly look amazing in a well-designed water feature. Many freshwater fish enthusiasts construct ponds specifically to meet the demands of koi. Although they thrive best in larger bodies of water, they are often reared in indoor tanks as well.
Available in a large variety of breeds, koi have preferences that depend on their mature size and density. Although they are considered highly adaptable, their basic needs have to be met as a means to achieve desired growth and survival rates. This means taking into account a host of tank limitations. Typically, a single koi carp would require a water volume of around 50 gallons or more, and this would be assuming a temporary (not permanent) tank accommodation period.
Koi can technically be kept in aquariums long term, but keep in mind that this may adversely affect their growth rate, lifespan and wellbeing. In a sense, it may be prudent to keep only young koi in tanks, with the intention of moving them into a larger pond after a few months. If you’re determined to keep your koi in a tank for a longer duration (but never permanent), you must balance your expectations and be ready to provide them with optimal conditions for maximum comfort.
DISCLAIMER
Koi require a large amount of water volume and space to thrive, and should not be kept in tanks permanently. The information here relates to rearing young koi or keeping koi in tanks for short periods. Koi should ideally be moved to an adequately sized pond environment when mature.
Introducing Koi to a Tank Setup
Koi are usually placed in tanks for a limited period of time. They are temporarily housed there for quarantine purposes, while draining or cleaning the pond, for aesthetic presentation in stores, or in preparation for transfer. Permanently keeping them in a tank setup is a whole other matter that requires thorough research and many considerations, and is not something we recommend for koi carp due to their size.
Ideally, you should introduce koi to a tank environment while they are still very young. Large, mature koi will always struggle to adapt to a more confined space, so make sure to use the largest tank/aquarium possible or even multiple tank setups.. If they are accustomed to outdoor conditions, a water depth of at least 3 feet, and a wide surface area, they will feel crowded in a tank. Young koi are more likely to adjust to tank conditions without experiencing significant levels of stress.
To safely introduce young koi to their permanent tank setup, they should undergo a brief acclimation period. This is a simple process that involves floating them in their individual, oxygenated bags prior to simply releasing them into the tank water. This prevents them from becoming shocked by differences in water temperature, light, and movement.
The setup should be placed in a quiet area that experiences minimal disturbances. It should be partially shaded, as direct sunlight can cause the water to quickly heat up and can stress the fish. Bright tank lights should initially be shut off. Initially, there should be no other fish in the tank as the newly introduced koi may carry diseases and parasites. When releasing newly acclimated koi, make sure that the tank is covered with a porous material (such as a mesh cloth). A canopy should prevent the fish from jumping out of the tank without blocking light and oxygen.
Tank Requirements of Young Koi
- Temperature: This should be maintained at 65 – 75˚F (18 – 24˚C) throughout the year. Koi will become stressed due to significant or sudden changes in temperature. Although koi in outdoor ponds can tolerate seasonal temperature changes, those placed indoors are less likely to adjust due to tank limitations (i.e. shallow depth, no specific areas that are either sunlit or shaded).
- Size: Young koi can comfortably be housed in 50 to 150-gallon tanks for a limited amount of time. Approximately 15 4-inch (10 cm) koi fish can be stocked into a 50 gallon tank, but they will quickly outgrow this and require a greater volume once they surpass a length of 6 inches (15 cm). If you intend to rear young koi carp in tanks for longer periods, a group of 4 will likely require a tank volume of at least 400+ gallons.
- Oxygen: The tank water should be aerated at all times. Dissolved oxygen concentrations should not dip to below 7 mg/L.
- Substrate: Koi aren’t too particular about substrate, but make sure to opt for one that won’t muddy up the tank water if disturbed. A shallow layer of clean pebbles, rocks, or gravel will do.
- Water: Tank water should be dechlorinated and filtered. Ammonia and nitrate levels should be kept at 0 ppm at all times.
- Filtration: The system should be compatible with the size of the aquarium and should ideally stimulate a current or light water flow.
- Lighting: If the tank is placed in a well-lit room, an aquarium light is not necessary. The fish should receive at least 8 hours of light and 12 hours of darkness each day on average. A standard aquarium lighting setup should suffice for aesthetic purposes.
- pH: 7 – 9
- Plants: These can be added to help naturalize the tank, cycle nutrients, and boost oxygen levels. Make sure to choose species that will not quickly outgrow the tank or reproduce via vegetative fragmentation. Keep in mind that these will have to be maintained and that koi may occasionally attempt to nibble on the stems and leaves.
- Tank location: The tank should be placed in an area with solid ground and direct access to a clean water source.
How Many Koi Can Be Kept in a Fish Tank?
There is no single answer to this question as variations in koi size, their growth rate, tank depth and surface area features, along with the fish’ purpose for being kept in the tank would influence the optimal number of koi. As a rule, you should never overcrowd a koi tank.
When purchasing young koi, try to find out as much as you can about their genetic lineage as this can help you gauge how large they will grow to be in a few years. It’s not true that koi will simply adapt to their environment and remain small if reared in a tank – this is an old myth! If they are cared for properly, large koi varieties will quickly outgrow a small tank.
To ensure comfort, a mature koi (10+ inches) should have at least 50 – 100 gallons of water in a fully controlled setup. Specifically, and to stock in conservative densities, every inch of fish should be accommodated by 10-20 gallons of water. Odds are you will have more than just one koi (they are not solitary fish and thrive best in small schools), so you must calculate accordingly. If you have a 150-gallon tank, you should be able to house a fair number of small koi (< 6 inches).
To house them for longer periods of time, you will need a much larger tank (upwards of 300-400+ gallons). If you have 4 koi that are expected to reach a mature length of around 16 – 20 inches (40 – 50 cm) each, you will require an enormous tank (around 1000 gallons)! That being said, many koi enthusiasts have successfully reared koi for extended periods in smaller fish tanks. Koi survival in these cases hinges on expertise when it comes to optimal filtration and responsible tank maintenance (water testing, proper feeding, disease control, regular water changes).
Can Other Fish Be Added to a Koi Tank?
Koi are peaceful fish that can get along with many other freshwater species in an optimal environment. They are generally friendly, and are thus susceptible to larger fish that may be aggressive towards them. They can be reared with an assortment of smaller fish, but make sure to choose species that won’t attempt to nip at their fins or vigorously compete with them for food.
Potential companions for your koi include goldfish, golden orfes, barbs, tench, small plecos, and hi-fin banded sharks. Some species of apple snails can make great companions as well and can help keep the substrate and glass surfaces free of algal growth.
As koi have heavy water volume requirements to begin with, adding other types of fish to your tank may do more harm than good. Consider the water volume requirements of other fish before purchasing them for the koi tank. It is advisable to choose equally peaceful, smaller fish that will place minimal pressures on the tank system in terms of their oxygen demands and waste production.
