I apparently they are fairly slow growers. But as I said 3' seems to be max size, but many guys said they have had fish in their koi ponds for 10 years and they are only 18". There is one guy who has several in his outdoor Idaho koi pond and they have done well for years. He is in Zone 4. The biggest concern I can see if that they escape from private ponds, get into a river here in North America where they can reproduce and badly affect the natural balance. However, I am not sure how they could be worse than Asian carp...
