scott, I also like non-fish in my planted fish tank.
thus far I've enjoyed; eels, frogs, crayfish, shrimp, crabs, snails,
but even I
would not buy a turtle.
under the assumption that a turtle prefers feeding above ground
rather than down in your water, you might, and I mean MIGHT,
get away with a turtle if you provided a large floating platform
complete with it's own grasses and leaves GROWING for him to
eat, however such a platform itself may block out too much light
that the submerged plants in your gravel will need to thrive.
maybe one of our terrarium regulars has tried such a setup