Adding "other" critters to my pond?
 

I am interested in adding some critters to my pond to add some additional
interest. I have koi and I like them but I would like some other
interesting critters as well. Frogs are an obvious choice but I am
interested in knowing what other critters I can add that are not harmful
plus will stay around.
I live in Oklahoma. Pond is large enough with waterfall that it does not
freeze over.
Turtles (small) - any problem? Where do I get the "right" kind?
Any suggestions or definite no-no's.
Paul