Koi are carp-fish and are admirably suited to the Swedish climate. There one can compete in Koi exhibitions just as we do with cats and dogs. Is all they need.
Plants give oxygen and shade and, at the same time, somewhere for the fish to hide themselves.
They eat most of what we eat. They love rice and corn. In that way you learn to get to know each other. You will quickly learn how pleasant it is to keep Koi. Once you start to keep Koi it will become like stamp collecting.