When you select the Swedish Fish booster, you will see the fish swim across the screen as the level starts – this lets you you know they're active.
To take advantage of the Swedish Fish booster, you must create new fish in the level by matching four candies in a square. When you create a fish, it will swim across the board and eat something on the screen at random, but with the Swedish Fish booster active, you'll create an extra bonus fish! What a catch!