* Many people would like to add turtles to their water gardens; usually
slider turtles.
* I would never mix turtles with prize KOI. Turtles will eat fish, if they
can catch them. Period. That's the way of Nature.
* However, in a large enough pond, I have successfully kept goldfish and
junk koi together with a variety of turtles. There are plenty of plants and
easy food for the turtles. The smart fish survive, the stupid fish get eaten.
So, I put in a few junk koi and a few dozen goldfish. After a few months,
the situation stabilizes.
* Turtles might also walk away from an unfenced water garden.
* Personally, I think one should either build a water garden and keep just
plants and fish; or one should build the pond with turtles as the primary
inhabitants in mind, and then it will be quite a different construction.
Note a few years later: I have a large (about 2000 gallon) pond where
I keep several large koi, large goldfish, and 5 large slider turtles.
Everybody is well-fed. The turtles have left the large fish alone for
the last two years.