Haha the shy one poking out from beneath the bridge.
Yes! A good rule of thumb is 10 gallons minimum per inch of fish. 25 cm is close to 10 inches and, which would require at least 100 gallons of water. 1000 liters/~250 gallons will allow the fish to grow. Make sure to keep the water clean and to change it often!
Fun fact- some koi fish breeders allot 800 to several thousand gallons to one koi fish alone to allow for maximum growth. Koi release pheromones into the water and when a threshold is met, the koi stop growing. This is why koi will generally only grow as large as the tank or pond. A way around this is frequent and substantial water changes!
Chagoi- the tea-koi. Named for their tea-like colored skin, chagoi are close to the common carp lineage. Chagoi are friendly, calm, and have a ravenous appetite, a perfect mix to create a gentle giant in any pond. Koi keepers often have chagoi in their ponds as a leader fish due to their size and calm nature, which is then reflected onto the rest of the school.
Koi fish can grow immense amounts given that they have the proper filtration and water capacity. We’re talking three foot long koi fish here! However, these “jumbo” koi are exclusively raised up by professional breeders. To give you an idea, one jumbo “show quality” koi requires two to three thousand gallons of water alone, as well as a state-of-the-art filtration system.
As far as having them in an aquarium, I see no problems with that as long as your filtration system is top notch and the aquarium is large enough. I found this water to koi ratio for the average koi keeper online:
Small Koi (2” - 8”): 100 - 150 gallons
Medium Koi(8” - 14”): 250 - 300 gallons
Large Koi(14” - 24”): 400 - 500 gallons
Jumbo Koi(24” - 36”): 750 - 900 gallons
Be sure to allot for their growth! They certainly grow, but something interesting is that koi give off pheromones ( a sort of hormone ) that chemically notifies other koi of their presence. At a certain concentration of these pheromones, koi stop growing. A way to keep up the growth in a smaller water capacity is frequent water changes.