If the amount of money reaches the nisab and one full Hijri year has passed, then zakah must be paid on it, whether it is for savings or otherwise. If the amount of money decreases and drops below the nisab during the year, then no zakah is due.

If the extra money earned during the year stems directly from the original amount, such as if the thousand was invested, and earned four thousand, then you should pay Zakah on the entire amount at the end of the year, because the profit on money is connected to the capital.