I didn't have a UV on the first pond but my koi are huge so I'll take any help I can get with extra nutrients (ducks).
Some people freak when a frog or toad gets in their pond. A koi with a healthy immunity can fight off a couple parasites like flukes. I have the microscope handy just in case. I've only had one snake. Don't mind them till I bend over and find it as a surprise.
What do you do for shade? I'm considering a shade sail, the koi destroy the water hyacinth.