Return to “20 Questions” page. A serious koi pond should be at least 3 feet deep and no areas should be shallower than 2 feet. Any areas shallower than 2 feet is an open invitation to dinner for both blue herons and raccoons.If you don’t think you have predators in your neighborhood just build a shallow pond and watch what happens! serious koi pond should be at least 3 feet deep and no areas should be shallower than 2 feet. Any areas shallower than 2 feet is an open invitation to dinner for both blue herons and raccoons. If you don’t think you have predators in your neighborhood just build a shallow pond and watch what happens!