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For inspiration, the developers should consult candy crush soda saga. The jelly fish are much more useful in that game, they seem to react to the requirement of the level. They also rarely hit boosters.
In candy crush, the fish only seem interested in clearing jelly. If that isn't a requirement in the level, then they are just a waste of time they avoid all frosting; they just clear random candies and are happy to ruin as many boosters on the board as they can find.
Something needs to be done to address the random nature of the fish booster too. If you play it, sometimes you may only get provided with one fish in an entire attempt at a level. Other times, you get loads and loads of fish. Using a booster should be less of a lottery