How deep is it ?
Any filtration system ?
Any oxygenation system ?
Is it shaded or in direct sunlight ?
Any pond plants ?
Offhand - without a seriously large filter system, or some tremendous depth (ie a huge water volume) - I suspect that that pond won't support Koi
well or for long.
A pond without a filter system of some sort (biological or vegetative) is basically one big fishbowl - and Koi
are tremendously messy.
Even 3 or 4 adult Goldfish
will probably severely tax the water quality of a pond that size if it doesn't have a filter.
If it gets some halfway decent sun, you'll have green soup and have trouble seeing your fish.
I would check your area yellow pages (or google) for pond supply shops (or aquarium shops that do alot of pond business) - different fish are legal (or illegal) in different states, and they'll know what a) is legal and b) will survive thru the winter (this will depend alot on the pond depth).
In my area, I usually recommend the pond be at least 3ft deep if Koi
are going to overwinter in the pond. I've had a number of customers over the past 10+ years who "got by" with 18-24" deep, until a bad winter a few years ago killed everything in pretty much all of the ponds shallower than 36" (and killed some fish in poorly maintained deeper ponds too).
Bare minimum I would add before adding Koi
would be a large biofilter (commercial or home-made) and an oxygenating water feature (waterfall, fountain, etc). I would also invest in the proper hardware for performing water changes - contrary to popular misconception, ponds need water changes just like aquariums, especially with Koi
. Other optional add-ons - UV sterilizers, bio-bead or sand filters, etc - would increase the odds of seeing the fish in the summer.
I would never ever ever recommend gambusia - "mosquito fish" for anything - they really don't eat many mosquito's, and they are a nasty little invasive species in many sub-tropical climes where they were introduced to eat mosquito's and proceeded to eat the native fish that were doing a better job on the mosquito's already.