Koi Ponds...

[naesco]

I have never had perfect conditions and I have never had a UV, filter or other stuff and I overfeed the koi.

If you have a lot of sun it will cause the growth of green algae.
In order to offset the sun and extra feedings, you need to have lots of plants that take the nutrients that the algae need to live on.
As well as oxygenating plants (the most important), you need plants that cover a large area of the surface of the water; Water Lilies and the tiny plants that multiply like crazy.
In the spring and when you first set up, the green algae give the pond a pea soup appearance. But, when the other plants take over, overnight the green algae disappear.

There may be circumstances when one or more of the equipment discussed might help the situation. All I can say is that I have never needed any of this stuff nor have any of the other people that keep ponds that I know needed it either.