Small koi will take mosquito larvae. I was surprised to hear a lady with middling sized koi (11-14") in her backyard mention that she had had to get some livebearers (red swordtails in her case) to eat the mossie larvae. Even though the koi are largely vegetarian, I would think that they might occasionally take a swordtail. But evidently enough swordtails survived that in the fall she netted quite a few out and took them to an independent pet shop. (The big box stores tend to be limited to the 150-200 fish on their computerized inventory and so their employees are forbidden to buy from aquarists.)
By the way that couple with the koi lived near a forest preserve. Her husband put up a 6 foot privacy fence around the yard and their two rottweilers to keep the racoons out.
One of the other correspondents mentioned Gambusia. Certainly if your pond was large enough and deep enough that the koi could over winter in the pond (with the help of one of those floating bubblers or heaters that keep an airhole open) the Gambusia might be a better companion.
Eastern Gambusia (either affinis or holbrooki) used to be put into waters around the world to eat mosquitoes. They still are by those ignorent of the danger to other fishes. While they certainly take mosquitoes, it has been found too many times that they prefer bite-sized fry of other fishes before they will take mosquito larvae. This has led to the extermination of populations and even entire species. So don't use them if flooding or water changes can take them to local bodies of water.
(Google "Dambusia")
If you are concerned about a few larvae making it to the midge stage in an area where mosquitos can carry West Nile virus, several kinds of encephalitis, dengue and yellow fever and most recently the Zika virus, you might wish to put something in that kills the larvae. Mosquito dunks look sort of like very hard little donuts and when dissolved in the water are supposed to kill gnat larvae like mosquito larvae,possibly blood worms and glassworms without harming fish or crustaceans. (All of those can be great fish food.)
The dunks can be had at pond supply stores, some garden centers and undoubtedly on-line. They are not inexpensive but one dunk (for a couple $) is supposed to treat 25 to 100 square feet of standing water according to the package we have. Moving water and fountains will also discourage mosquitos from laying eggs (which look like little pieces of charcoal scratched out with a fingernail).
In the past I have raised Daphnia outside to help feed fry and small breeding fish. Some larvae escape the filter feeding Dapnia and I tried to catch most of them because sieved for size, they area terrific fry & fish food. There are ethical reasons to not raise them so I'd destroy (feed to fish) all I could. Occasionally a bicket was just dumped.
Some municipalities ban open water containers. (Not ours...yet) My somewhat iffy rational was the mosquito larvae harvested from a back yard tub are mosquitos that otherwise would hatch and grow in nearby wet lands.
If you had what felt like flu for a couple of weeks in the 10 or so summers past (I did), you probably already have had West Nile and are possibly now immune to it. But this Zika Virus worries me a lot, wondering about the threat to unborn grandchildren in our family or anyone else's babies.
I'll probably continue to raise fish, water plants and Daphnia (if they survive the dunks) outside this summer. But those mosquito dunks will be used with those water containers and the water surfaces will be closely watched.
See the last source below, just as a start on that topic.
I fear that with the blowing of winds and strong storm fronts, much of the US may have mosquitoes brought in that might carry Zika virus.
So thank you for your question. Hope you enjoy your koi, but are careful with your pond and successful in eliminating mosquito larvae.
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