Mosquito Torpedoes.
The active ingredient in Mosquito Torpedoes (S-Methoprene) is an IGR (Insect Growth Regulator) that affect mosquitoes at every life stage prior to emergence eliminating their ability to develop to the next stage thus never becoming adults. The ingredient is very specific to Mosquitoes and does not harm any creature with a spine. So it's safe for people, livestock, fish, frogs, birds, etc. Torpedoes are approved for use in stock tanks in the US. In many tropical countries with severe mosquito (and hence malaria) problems they are used in human drinking water.
Five Mosquito Torpedoes come per package for $8.99One little Mosquito Torpedo will provide protection for 60 days in up 100 square feet of water 2 feet deep (1500 gallons). If your pond is larger put two or more Torpedoes in. If you put them in a smaller tank, such as a horse trough, bird bath or a smaller pond the Mosquito Torpedo will last much longer, perhaps all summer. The Mosquito Torpedo gets smaller as it slowly releases S-Methoprene. If you can still see it, it's providing protection. Another advantage over hardware store dunks it that the pond or tank can dry out, and if the Mosquito Torpedo is still present, it will start protecting again when the tank or pond refills.