A 120 to 180-cm-long aquarium is quite a large aquarium to keep koi in. You can place the aquarium on your porch and keep 5 to 10 koi of 30 to 50 cm long, enjoying the true value of koi.
With this method, you should be careful not only of the water quality but also of the water temperature. The requirements for a large aquarium are the same as a small one.
Electricity charges can be quite high to maintain water temperature. You can cover the aquarium or lower the water level so that your koi won't jump out of the aquarium.
The maintenance of the aquarium and feeding amounts are the same as keeping in a small aquarium.
Koi of 30 to 50 cm long will easily become tame.