They will only get 20 inches, but thats not good, they will be stunted, probably have a crooked back, and they will be highly inclined to jump and try to commit suicide because they probably want to die. And the only way you could prevent them from doing it is by forcing the lid down with bungee cords and bricks, but then you are pretty much keeping an animal against his own will IMO.
Just get a bigger tank.