So you have a lovely watergarden, shallow and full of plants, or are planning one, and have come to the realization that it is not the correct environment for Koi. Koi require a certain type of pond to thrive and will eat any plants you put in with them.
There are alternatives for pond fish that are just as attractive as koi and can bring as much pleasure (and hard work) as a koi pond. Shibukins, commets, and orandas are hearty goldfish species that are easy enough to find. They come in a variety of colors and can add that extra touch to the water garden. However if you want a fish similar in appearance to koi, the Japanese Wakin is a lovely breed which is shaped similarly to koi but has a split tail. It is mostly red and white so it resembles a Kuhaku. The body is deep but overall length and width is much smaller than koi. Wakin should be viewed from the top so are perfect for an in-ground pond, rather than an aquarium. They are easy to keep and are docile. Wakin will eat out of your hand. Wakin are cold hardy so can be kept in colder climates.
Water quality and filtration is a must if you want your wakin to grow large (18+ in.) and healthy . They should be fed quality low fat goldfish food , not high protein koi food, and if underfed will eat your plants, especially lilies. Giving them plenty of room to grow also helps. Plan on a minimum of 20 gallons to 50 gallons per fish. And since they are so colorful, you will need predator protection like netting or fishing wire around the pond.
Wakin are relatively rare in the U.S. So if you can breed them you may find you can offset some of your pond costs. Here in the Atlanta area you can find them at Kolkoi and Wakoola.