Koi will reach a length of a foot or more during the first year or two of its life and will continue to grow for the next decade. By the time it reaches it full potential , it will weigh more than your average house cat. So if you can imagine two house cats swimming in a 29 gallon aquarium with the ryukin, the picture is not very pretty. You would need an aquarium several hundred gallons in size to be able to successfully raise the two koi fish, thus the reason most koi fish are limited to the outside pond environment.

Goldfish and koi can coexist just fine, but you will likely have to upgrade your tank to a bigger size if you want to keep the koi fish indoor . Koi can grow very fast so a 29 gallon aquarium will not be sufficient for very long.