I have a koi pond at home.  Koi are considered colored varieties of Amur carp.
Oh interesting, Ive been actually wanting to get some for my small pond, Are they hard to take care of? 
Not really.  They are coldwater fish and they have a great ability to adapt to many different climates. 
Oh very nice, I suppose they would be a good fish choice then, How big do they generally get?
Koi
Koi is an informal group of the colored variants of "C. carpio". Several varieties are recognized by the Japanese. Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, orange, yellow, blue, and cream. The most popular category of koi is the "Gosanke", which is made up of the "Kohaku", "Taisho Sanshoku", and "Showa Sanshoku" varieties.
History.
Carp are a large
group of fish originally found in Central Europe and Asia. Various carp species were originally domesticated in East Asia, where they were used as food fish. Carp are coldwater fish, and their ability to survive and adapt to many climates and water conditions allowed the domesticated species to be propagated to many new locations, including Japan. Natural color mutations of these carp would have occurred across all populations. Carp were first bred for color mutations in China more than a thousand years ago, where selective breeding of the Prussian carp ("