i would never keep any fish with turtles that i was not prepared to lose. the turtle would probably be safe from attack in its shell provided it was too big to fit in the fish's mouth, but down the road, even a small turtle can take a notch out of a big fish, prepare yourself for the inevitable injuries from the reptile on your fish. i would not attempt it unless perhaps you had some larger fast moving species that the turtle would unlikely be able to catch. oscars on the other hand... while larger... are not at all fast moving and i think would eventually get nipped to death by a turtle. i used to keep a map turtle in a 30 gal tank with 2 goldfish when i was a child, and it would chase those goldfish around 24/7, eventually killing them both, even tho the goldies were larger than the turtle. you will need some powerful filtration for the turtles, EXTRA powerful if you intend to keep fish in the same water. also... i would not even attempt a dual habitation unless it was a LARGE system. turtles can be relentless when they are hungry, your fish needs plenty of room to get away. i dont care how cute that turtle is, it will KILL to feed, for it is a reptile, and thats what reptiles do. so expect no friendly co-existance, anything that turtle can get in its mouth, will likely get bitten if it gets the chance to do so.
