Fish varieties that can be put in a water garden?  

I've heard lots of people talk about how they kept guppies and other livebearers or mosquito fish outside in ponds. But, I don't think they'll go well with the koi, think the koi would eat them. And, I think you'll have to bring them in if it gets cold in the winter. I don't think you're going to have much luck, maybe a fancy goldfish variety, but they're not yellow. Sorry, good luck!

One of my outdoor ponds has mosquito fish and they live year round in the pond. My area doesn't get cold very often, so tropical fish can live in a pond, but if your weather is colder than mine I wouldn't recommend having tropical fish in a pond.
You mentioned Koi are in the pond now and I'm positive, if you put mosquito fish in the Koi Pond they will be eaten.
I don't know if there are Golden Orfe fish available in your area, but you might consider them. The fish is a yellowish-orange in color and are cold water fish. Golden Orfe's are a schooling fish, so if you decide they are perfect for your pond get at least three and they like to play by jumping, so are best in 500 gallon ponds and larger. Good luck and hope your able to see these fish.

Hi there, I hope I can help. I have kept stickleback fish in my aquarium, and they can even stand the winter cold. They are coloured red and some species have a yellowish tinge to them. Maybe Goldfish would work for you as well, some of the bigger species, such as the Bubblehead can withstand pond temperatures, and they can grow to pretty formidable sizes ( I have seen 14 inch goldfishes )

The colouration on goldfishes is very pleasing to the eye, I'm sure you would be able to find varieties in orange, yellow and red. Happy 'ponding'!

many can be but you need to maintain for that kind of fish