we once purchased a tank which was 12 months old. It had a shark,angel,firemouth, and a number of tetras along with a red eared turtle.
these had all got along famously until the turtle realised that anything else in the tank was meals on fins. He started with the shark and moved his way down by size, we had to remove him quickly.
On saying that I have a 6ft tank with 5 turtles which I try to feed guppies but the turtles just let them breed and only eat the guppies when they have died.