The appropriate size of a koi aquarium should be determined comprehensively based on the koi's growth stage and the number of fish being kept. The minimum recommended size is 1.2 meters in length, 0.8 meters in width, and 0.7 meters in height or more, to accommodate the adult koi's body length requirement of 50 - 80 centimeters. The detailed analysis is as follows:
I. Choosing the Size Based on the Koi's Growth Stage
Juvenile Stage (Body Length: 10 - 15 cm): The minimum requirement is an aquarium that is 60 cm long, 30 cm wide, and 40 cm high (with a water volume of approximately 72 liters), and only 1 - 2 fish can be kept. If you want to keep 3 - 4 fish, it is recommended to choose an aquarium that is 80 cm long, 40 cm wide, and 45 cm high (with a water volume of approximately 144 liters).
Sub - adult to Adult Stage (Body Length: 20 - 30 cm): The minimum requirement is an aquarium that is 1 meter long, 50 cm wide, and 50 cm high (with a water volume of approximately 250 liters), which can hold 2 - 3 fish. If you want to keep 4 - 5 fish, it is advisable to select an aquarium that is 1.2 meters long, 55 cm wide, and 55 cm high (with a water volume of approximately 363 liters).
Adult Stage (Body Length: Over 40 cm): At least an aquarium that is 1.5 meters long, 60 cm wide, and 60 cm high (with a water volume of approximately 540 liters) is needed for 1 - 2 fish. If you plan to keep more than 3 fish or mix them with other species (such as goldfish), it is recommended to choose an aquarium that is 2 meters long, 70 cm wide, and 70 cm high (with a water volume of approximately 980 liters).
II. Choosing the Size Based on the Number of Koi Being Kept
Follow the principle of "keeping 1 cm of fish per 10 liters of water": For example, a 500 - liter water volume can hold a maximum of 10 koi with a body length of 50 cm. In case of overstocking, immediate aeration is required, and the fish should be transferred in batches while starting a high - power filtration system simultaneously.
Consider the size adaptation of the koi: Large - sized koi (with a body length over 40 cm) need a horizontal space of more than 1.5 meters. For small to medium - sized groups, the length of a single tank can be reduced appropriately, but the total water volume must be ensured.
III. Other Considerations
Reserve Growth Space: Koi grow rapidly, with juvenile fish able to grow 10 - 15 cm per year. The depth of the aquarium is recommended to be no less than 70 cm to reserve activity space for the adult size of the koi.
Choose a Rectangular Aquarium: A rectangular aquarium not only has a large water volume but also provides sufficient swimming space for the koi, and the filter can function better. Avoid choosing circular or irregularly shaped tanks, as these types of tanks have a small actual water volume and are not conducive to the koi's swimming.
Consider the Space Occupied by the Filtration System: Koi require a powerful filtration system. It is usually recommended that the water volume of the filtration system accounts for 1/3 to 1/2 of the total water volume. For example, for a 1 - meter - long aquarium, the filter tank should be able to hold at least 80 - 120 liters of water.