Yes, most of us know Mosquitos are attracted to Stagnant Water to lay their eggs. The eggs soon hatch into larva, which most of us have seen in water. A properly designed and functioning water feature will have enough water flow to break the lifecycle of the Mosquitos by tossing, tumbling and drowning the mosquito larva. Most pond fish including Koi will consider any bug including Mosquito larva a delicious meal.