I love koi !! I didn't know you could keep them in a tank though... . We have a large pond in our garden, with lots of koi, Japanese and Israeli(ghostkoi). They are all so fantastic!!
This is a link to my website, to the page with pics of our pond.
http://users.telenet.be/bernardgabriels/thepond.htm
And here are 2 good links;
both links are in Dutch, but you can see all the pictures!!
http://www.dekoi.info/soorten.html

http://www.ottemadokkum.nl/soorten/#top