We have always understood this and worked hard with koi keepers to provide the right conditions in beautiful surroundings that show off healthy fish at their best. Sometimes meeting these conditions requires complex solutions.
Recently, a client wanted to show off his fish in a pool at the bottom of a large waterfall.
Keeping control of the water conditions for a relatively large number of fish was to present a challenge. We did it. The water from the waterfall fell into a hidden pool just behind the koi pool. The water from the koi pool was constantly gravity filtered and returned to the pool through jets hidden in the rocks edging the waterfall return pool. The illusion was complete and the fish got the best living conditions.