Turtles, in a natural pond, eat small fish, dead fish, plants, snails,
insects, worms.
I've kept turtles in a 3,000 gallon pond. They would share the fish
food I fed the fish. I have an island in the pond where I would toss
extra fish food and land snails that I found in the yard for the
turtles.
I kept extra floating plants in the pond for the turtles to eat
(water hyacinth) and I was not concerned about any damage they did to
the lilies (figured it was the price to pay).

I did run a filter on the pond.
Hope that helps.

kathy
www.blogfromthebog.com