Turtles are tricky in ponds...
They eat everything (IE fish, snails,and will snack on plants). They are good exscape artisits. And as Joe pointed out, if you want to over winter them it is almost next to impossible due to the need for something to burrow into. I put my E. Painted in my pond in the summer, but only for a few hours at a time, while supervised (sitting on the patio enjoying a frosty bev.) He likes to chase the fish and get some real sunlight to bask in. I like so he can stretch out and really swim.