Pet-shops where you buy your Koi will have good books on the Koi-keeping. Once you start to keep Koi it will become like stamp collecting. One simply has to have more and better specimens. Do not buy too big fish. They grow quickly. A Koi can grow to be over a meter long. They grow according to the space you can give them. If your pond is sufficiently deep your Koi can remain in there over the winter. They become dormant and require no food