Some fish species are actually harmful to the environment. In the Nordic countries they often try to get rid of the roach, which muddies the water and destroys the habitat of more worthy fishkind.
This is complete nonsense.
If there is any species that is "harmful to the environment" it is humans.
Thou knoweth nuffing. The lake turns into smelly soup harboring poisonous algae, which kills indigenous species, like easter bunnies and red-nosed reindeers.