Where you have turtles, you probably have some frogs. We once had a turtle pen beside a stream.
The frogs were very numerous and aggressive. If we dropped a worm in front of a turtle,
a frog would emerge from the grass and snatch the worm before the turtle.

Turtles and frogs compete for the same food and the frogs are generally faster.
Well fed turtles still enjoy bugs, slugs, worms, spiders, ants, etc. Frogs eat anything they can
get in their mouths and they have big mouths. A big frog will eat smaller frogs and baby turtles.
So we will tolerate only one or two small frogs if they find their way into one of our turtle pens.
We remove large frogs. That is not to say that we don't enjoy having frogs around but not in our turtle pens.