Guppies have been deliberately set free in Asian waters to fight the spread of malaria. Malaria is passed on to humans by mosquitoes, but guppies eat mosquito larvae, which helps reduce the number of mosquitoes and slows the spread of the disease.
Actually, from what I’ve read they had no discernible impact on the mosquito populations. In fact the only impact they had was a negative one on the local fish populations. (source)