Probably not. I used to have red eared sliders in an indoor pond with 2 oscars, and they did fine, but the turtles did, on occasion, try to take a bite out of the oscars. It was only the size of the water garden that saved the oscars. Also, turtles produce a lot of waste; probably twice as much for each turtle as an adult oscar. A third thing to consider: turtles require a resting ledge or platform out of the water, and a heat and UV
lamp to bask under, unless they get ample natural sunlight. If they don't get sufficient UV
rays, they won't process the required nutrients and will waste away and eventually die.