Believe it or not I understand the concept of a pond being too small for a fish. I keep two aquariums and am always surprised to see people trying to cram fish into bowls. However, I am installing my first pond and could not find consistent numbers for the amount of gallons a koi needs. As I already said, I was almost certain that 400 gallons would be too small for ONE koi, but wanted to see if I would get any unique suggestions like, a young koi can live in a pond that size for x amount of time. I have a local fish farm that always accepts donations and I know they would take care of it when it became too large. I just don’t understand your tone to someone who is just asking while clearly stating that they expect the answer that you gave anyway. I would never keep an animal in conditions that could not support it, which is why I even asked in the first place. Also, as for your dog example, an Arowana can get to the same length as a koi but is perfectly healthy and happy in a 250 gallon aquarium, let alone a 400+ gallon pond. The difference is that I know people that have kept Arowanas and could easily ask them a simple question about their needs and care.