Eduard, welcome to Pond Boss!

(Also, good job getting your own thread started.)

Is your pond mostly ornamental? Is there any fishing allowed?

One problem may be too many fish! As the fish (all species) grow, some must be culled to maintain adequate space in your pond.

Further, if people feed the koi, then there is a huge buildup of nutrients in your pond ecosystem. Even if your beneficial bacteria is helping breakdown the muck, those nutrients are still in the system.

You probably need to pull some sample jars to determine exactly what is causing your murky water.

It could be actual silt/sediment stirred up by the fish. It could be blooms of phytoplankton (algae) and zooplankton. It could be clay particles suspended by their electrical charges. (There is a treatment for that condition, that will not harm your aquatic creatures.)

Looking at your pics, have you considered adding some floating islands? (There are some good threads on Pond Boss.) The plants on the floating islands will use up some of the excess nutrients in your pond and make less available to the algae. Further, the turtles like to loaf on the islands - and the people that like turtles would probably get some enjoyable photos.

Good luck with your big pond!