I have Koi as well, and keep them in main pool. The disadvantage to Koi is that they need a lot of room, primarily surface area more so than depth. You will need typically at least 2 ft of depth depending on climate. Koi get large. my breeders are over 2 ft. And I get spawnings from them 2x a year. They make thousands, and if you use spawning mops you can isolate the eggs from them as they WILL eat their own eggs as well as each others.

Butterfly and Japanese are two types that there's always a market for. Within that, pick a "type"....usually refers to patterns style to go with. DO NOT mix the patterns or your $ value drops dramatically. That's what seperates a $1,000 Koi from a $20 Petco Koi.

Go to 1/2 price books, or amazon, or whatever and get yourself a book on the different types of koi.