18" long and it lives fine.
If this is true, the fish is already suffering, you should never keep a fish once it gets as long as the tank is wide. It cannot turn around naturally.
I'm locking this thread, it's already getting uglier than need be. While I am as disappointed that someone on this forum has such little reguard for the life they keep in their tank, it is obvious that he is going to do what he wants anyway. There is no need to go through the ugliness.
Remember, you can lead a horse's *** to water...but you can't make him drink.