I have a full grown wild caught Midland Painted Turtle. He's mild-mannered so I did try dunking my betta in with him. All the turtle did was his little hand wiggle thing that he does whenever I put something new into the tank. The betta didn't really seem to care. All they did was keep to themselves aside from the occasional poking from my turtle. I'd say if that you put the betta inside the turtle tank while the turtle is still smaller, they'll get along fine in the long run.
If the turtle is showing any aggression towards the betta, try putting in a divider or even something called "betta barracks" so they can get used to seeing each other. If that doesn't do it, keep them in separate tanks.
If the betta and the turtle do end up getting along, find a happy medium amount of water for your betta and your turtle. Since you didn't specify what turtle you're getting, my theory might be a little off. My turtle is a midland painted and I keep the tank filled all the way with three to five inches of air at the top of his 60 gallon tank. He does just fine with that. I just threw in a floating peice of driftwood from the beach so he has a place to rest and bask while the betta still has the rest of the tank to swim around in.
If the turtle is showing any aggression towards the betta, try putting in a divider or even something called "betta barracks" so they can get used to seeing each other. If that doesn't do it, keep them in separate tanks.
If the betta and the turtle do end up getting along, find a happy medium amount of water for your betta and your turtle. Since you didn't specify what turtle you're getting, my theory might be a little off. My turtle is a midland painted and I keep the tank filled all the way with three to five inches of air at the top of his 60 gallon tank. He does just fine with that. I just threw in a floating peice of driftwood from the beach so he has a place to rest and bask while the betta still has the rest of the tank to swim around in.