State officials say while releasing fish bought in pet shops may seem harmless, they can become invasive and multiply by the thousands, or they can struggle to survive and to fend off more aggressive predators.

Seth Herbst, a biologist with the department, says while Michigan hasn't yet suffered "detrimental impacts" from releasing pet fish, the potential is there.

Reduce Invasive Pet and Plant Escapes, an environmental group, says owners should instead donate the fish to a learning center or zoo.