Considering that with proper care, koi can and regularly do exceed 2ft and can reach well into 3ft in length, I think it is abhorrent to keep such an active, high-waste fish in small quarters like a tank under 300g. They are pond fish, pure and simple, if you intend on keeping them healthy and happy for the full duration of their lifespan and maximum potential of their size (unless you're one of the lucky few with a several thousand gallon tank). Part of ethical fishkeeping is providing for your fish's needs, not seeing how big of a fish you can cram into how small of a tank just for the sake of owning it.
I wouldn't even keep single-tail goldfish in a tank for any reason but growing out or overwintering; I had two foot-long common goldfish in a 75g, and not only was it a struggle to keep clean with great filtration, but every time they spooked, darted, tried to play, tried to jump, etc. they hurt themselves because the tank was simply too small for them to move about freely in. I now allot 100g per goldfish (I have three in a small pond, all males so that's it!) and they're extremely healthy, have great musculature, and have had very impressive growth rates.