I maintain about 20 Koi pond here in San Diego from a 5k to 15k gallon, from the most cheapest simple to the most expensive filtration, I have an OLD saying do it right and you will have once ask away and I'll try to help
They get big, so lots of room to swim is a must. They need good filtration to survive. Other than that they are the coolest fish out there. Nothing like having a fish so tame you can hand feed him and pick him out of the water.