Ditto with what everyone else is saying. I wouldn't be to concerned with the turtle. Unless you put him on speed there's no way he's gong to catch many minnows.
I had seven cages of fish in one of my ponds and was initially concerned the turtles might chew their way in, and inadvertently allow fish to escape the cages. They did seem to be hanging around the cages. It had me a little concerned as I went to a lot of trouble creating a male only bluegill and female only perch pond, with female bluegill and male perch in the cages. Turns out my fears were ungrounded as when the caged fish were moved and the cages removed from the pond there was no damage whatsoever.
I think the turtles hanging around the cages had more to do with the stray pellets vs. the actual fish.
