To keep Koi fish in an aquarium, you want to try to replicate their natural environment, just like you would do with any other fish. Koi fish require very clean fast-moving water, highly-oxygenated water, and they need a very large environment. They get very, very large. Even though freshwater fish, their size will be limited to the size of the aquarium, their growth will be very stunted; their gill plates will keep growing, their fins will keep growing. They're going to get a hunchback appearance to them. You want to go with a very large aquarium. I don't recommend Koi for a small aquarium; when I mean small, anything less than 200 gallons. The Koi will just outgrow that, they're not going to be happy in there.
If you are going to keep Koi in an aquarium, you want to make sure that you have very, very good filtration; strong, fast-moving water, highly-oxygenized. You want to keep the pH between 7.2 and 7.6. They like slightly basic pH. You want to feed them very good foods, vegetable-based foods. You also want to maintain the water temperature where they need it to be so their immune systems can function properly. They do not fare well in warm water, so it's best to run a chiller on a Koi aquarium. You want to keep the water temperature to low to mid-60's. That is where I feel they do best. The oxygen levels will also stay higher by keeping that water temperature a little bit cooler.
You do not need a lot of decorations for them. They do need some areas where they can hide when they feel a little scared. They are pretty open, fast-moving fish so give them a lot of room to swim in, and don't clutter the aquarium with decorations. They're not like Jungle Fish or Reef Fish that spend all their day hiding in a cave. They are out cruising around, scraping algae off of rocks. You want to have a nice, clean, open environment for them.
If you like fish that look like Koi, maybe go with goldfish, Comets, Ryukins; any of the . . . they're smaller, they do better in an aquarium than Koi. Koi, I feel, do best in an outdoor pond setting.