I've kept turtles before.
I always tried to think about what they ate
in the wild and took it from there.

Young turtles need more protein, to grow
their shells. Older turtles need more vegetable
matter.

We have an island in our pond. I tossed extra
treats onto the island where the turtles could get
it and not the fish. Stuff like ~
land snails
worms
organic lettuce
watercress
and turtle chow when I had it. I figured there were
some goodies in there they could use but I didn't go
out of my way to make sure I always had it.
I've heard that they like fruit also.

That said, the turtles were always leaping into the
skrum of fish and giving no quarter going after
fish kibble. I'd throw a bit of that in their island also.

k :-)