Nope, it would be nearly impossible. Koi are coldwater and rays are tropical. The ray can't handle temps that low and vise versa. Not to mention kois have HUGE bioloads and produce alot of waste. And rays are SUPER sensitive to ammonia poisoning, I learned the hard way... Rays produce enough waste by themselves. What are you planning to do? Do you want to keep them in a tank or pond? And do you want to keep them in the tank for the winter? If you live in a warm enough area such as in florida I would get some MONSTER fish as a perminant pond such as red tails and/or arowanas . But that's me lol
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