That's can be true for babies, but for adults the more space the better as long as you fill it up with decor that will make the feel secure. I have two smallish males in a 50 gal. long and they love using the whole tank. If she's a yearling, she'll probably keep growing till she's three years old but by the time they're 1 1/2 to 2 years they've usually outgrown the skittish, insecure baby phase in my experience.