|Is there a minimum requirement for the depth of a pond that contains koi? Our pond is 18" deep, 10x5 feet, and well shaded.
The answer to that question depends upon the severity of your winters, whether there's ornaments or rock on the pond bottom and whether you want big Koi.
If winters are severe, the pond could freeze to the bottom. Or close to it, and that would wax some Koi.
If the pond bottom is abrasive, or rocky, the fish, as they grow will develop contact sores on their pelvic and anal fins - possibly but not always leading to ulcers. Smooth gravel on the bottom of the pond (the only kind they use unless they WANT holes in the liner, is not a problem. Just try standing on it, to consider practically how slick and NON ABRASIVE it is. Don't get all excited about gravel bottom ponds. Just keep them clean.
If the koi get really big (16+ in.) , they will begin to "stress" about the shallow depth, not only because it violates their sensibilities about predators but also they will scarcely be able to swim 'up' or 'down' in the limnion.
While I've seen tons of happy 24-36" deep ponds, I have to say the "perfect" pond, in my experience, was 4 feet deep. You can keep good clarity in that without a lot of trouble and the fish are happy with it. In severe climates, six to eight feet are supposedly mandatory. Uh huh. I had a pond eight feet deep and clarity to the bottom was slightly challenging, catching fish was a challenge and I had to turn over all that "bottom water" to keep oxygen tensions high enough.
NO POND DEPTH AVOIDS PREDATORS. Predators like Herons, River Otters, Pelicans and Cormorants do NOT CARE how deep the pond is, or isn't and can clean out the pond. Especially River Otters.
Beavers, and Muskrats are vegetarians. Mink don't prefer fish but will take them. A Snake or a bullfrog will take a small fish easy.