Koi need room (large, deep ponds) and filtration systems, and can grow quite large in the right conditions. They can make a mess by knocking over plant pots, and sometimes eating the plants. Alternatives for those with smaller ponds include Sarasa Comets, Shubunkins, and common goldfish.
These are just a few of the options you have under the Fancy Goldfish category:
These are just a few of the options you have under the Fancy Goldfish category:
Visit the Bristol Aquarists Society page for information on these pretties, and many more.