I have three ponds.... One for the koi and lilies, and, in a seperate and securely fenced area, two for the turtles. Turtles will eat your fish if they can, so seperate quarters are a good idea. Also, turtles are much more dirty than even koi. With the fish, I simply clean the filter and do a partial water change each Sunday. But with the turtles, I completely drain and refill the ponds every week - and the pond with the most turtles gets drained and cleaned twice a week during warm weather (when they eat and poop more).
I know it's more work and more space and more things to buy, but consider having seperate quarters for fish and turtles.