They will always be on the nervous side. It's their nature. But the bigger and older they get, the less of that you will see. Still, it will never go away.
The best practice to alleviate that is to have a large tank (they require a bigger tank than the 4xL by 2xL by 2xL rule of thumb calls for) because they swim constantly, not to have anything sharp or abrasive in the tank that they can injure themselves when they do dart, have friendly, non-aggressive tank mates, and try not to spook them.
