The surface of the bottom of the pond probably already houses a lot of nitrifying bacteria from rotting plant matter, etc... I bet a few fish wouldn't change much, but I guess it certainly is a possibility. Platy will eat the larva, but they certainly are not the best mosquito eaters!
Thanks for the responses. yup the pond has anacharis, cabomba and floating plants like pistia and the fish are doing fine because the water never clouds or has any fatalities. Im also doing regular water changes.