This lifetime exchange of different substances between the koi and its environment goes on through a semi-permeable membrane. Semi permeable membrane stands for that a few substances only can travel unidirectional through the membrane.
Koi can not exclusively select which substances it will permit to pass across the membrane. This semi permeable membrane has been evolved over millions of years and the development has came about in natural rivers and lakes and not in ponds and koi water which means this process has assumed the conditions to that of lakes and rivers. Surroundings in rivers and lakes are well balanced eco systems due to their large size. Large variations in concentrations do not crop up. If it happens then it would be in a very gradual process over decades that allow the koi to adapt or die. But, in a pond conditions can swing decisively in as less as 24 hours leading to destruction and death.
Fluids tend to mix each other whilst they are in close contact. The fact is such that the semi permeable membrane which separates fish from its watery environment cannot avert a case of diffusion. Diffusion is a process which permits the bodily fluids of koi to be greatly influenced by the outside water. So, if you change the habitat, you change your fish. The greater the size of the pool, the more stable it is prone to be. Fish only can adapt to changes in their atmosphere in a controlled fashion and over a small range or tolerance. You may have to face these challenges while keeping koi in a pond. The maintenance of koi water should be stable, predictable and consistent.
There is no difference between koi and other fishes. They need foods just like other animals in that they require vitamins, proteins, fats, minerals, carbohydrates etc.
Since they are in continuous contact with watery environment they also require dealing with the constant osmotic and diffusion processes that happen in watery medium. Osmosis requires energy. It is the movement of water from a place of higher concentration to lower. Energy involves oxygen. A fish also generates its own body heat besides osmosis. Koi simply could not get enough oxygen from water to execute these processes. They are also relied on oxygen from the air.
It is crucial to know that every water substances and properties (such as pH and temperature) in Koi Water tends to be at equilibrium and the state of equilibrium seldom happens in Koi water, but it is very important to note that when it happens things tend to be very hard to get there.