Hi Claude
I wouldn't do it. Turtles eat everything! They tear up your plants
and do eat fish if the can catch one. Mostly lazy, if food is
plentiful they won't eat the fish but they're not shy about nipping at
them. At the least your koi will have shredded fins and bruises, but
if the opportunity arrises, Shushi is served. Plus they make a holy
mess with all the waste. Not worth it. Most people have a seperate
turtle pond.
On the other hand, and OT, tortoises are the best! We have two desert
tortoise in an enclosure in the backyard and they are great. In
Arizona, if they are born in captivity they cannot be released into
the wild and need to be adopted. Ours are two years old and are great.
Since they are endangered, it is kind of cool to be responsible for
them. Of course, I have to keep them away from the pond too cause if
they fell in they'd sink like a rock!