That is a beautiful pond!
Right now I just got a koi pond, last year the adults just.started.dying, all within 2 weeks, no water problems or nothing, but luckily they had.spawned and there is about 100 goldfish. Sized guys in there
My biggest koi are about 7 years old . I've never been able to get babies, between the koi and the turtles the eggs are gone with in minutes of being laid .
A couple of ours.that died were pushing over 13yrs old it was sad! They spawned just about every year, since all the adults died these koi didn't have much danger