Ether Koi is a unique collection of 5,000 Ether Koi NFT’s inspired by the most expensive pet fish in the world, Koi. Koi fish can be worth millions — the most expensive koi fish ever sold was worth $1.8 million. Koi are a type of carp, a common fish that can be found all over the world, but what makes Koi so special is their coloring and lineage.