There are thousands of two-foot deep koi ponds full of happy and healthy koi.
A common myth is that ponds must be built at least 3-feet deep, especially if they contain koi and or are located in a colder climate. You see, the water in a two-foot deep pond will generally only freeze eight inches down, even in the coldest of climates, because of the insulating qualities of the earth that surrounds the pond.
A garden pond that is too deep could be considered a swimming pool by your local government and therefore fall under strict guidelines and codes. Also, more digging means more work, more water to fill the pond, and more additives to treat algae and fish illnesses.