There is no need for chemicals, I have 3 totally easy, natural and effective ways to keep mosquitoes from using your water features as a breeding ground.
Water movement keeps them from laying larvae. Installing a small fountain or bubbler will do the job. If you have a container water garden, water feature or bird bath, simply over-filling the container with water daily will wash out mosquito larvae. Mosquito fish (Gambusia Affinis) will also eat the larvae, as will tadpoles I believe.
For larger ponds you can use mosquito dunks. They use a bacteria called B.T. (bacillus thuringiensis), this bacteria is harmful to the mosquito breeding process but harmless for children, pets, birds, plants, or any fish you might have. They also control gnats and can be found online or at any garden center. For small ponds you will use a whole mosquito dunk and for small containerized water gardens cut or break the dunk into one-third or quarter-size pieces.