According to a rule of thumb, keep your tank to a ratio of 2 gallons for every inch of fish you have. So if you a have fish that is 3 inches, then you would need 6 gallons of water for it. But you have to consider that the fish will grow so you will need to accommodate. As for koi, koi can grow up to 2 feet in length or even longer in most cases. Koi should only be kept in ponds. I have a 6000 gallon pond and the koi are amazing.
