Do you know the reason why a koi pond in general is 4ft deep? One of those reasons is climate. Here in Puerto Rico it’s never colder than 20-25 degrees Celsius. This pond is a converted koi pond, maybe in the future he will get a new pond. For now the filtration, bottomdrains and 10K liters of water is enough to hold the fish
Yvo - you should know that such shallow ponds will only result in the koi having poor body shapes with pot bellies and no muscle as they cannot go up and down to the depths.