To prevent obesity in pet turtles, it is important to provide them with a balanced diet that is appropriate for their species. Avoid feeding them excessive amounts of high-fat or high-carbohydrate foods such as commercial turtle treats, bread, or fatty meats. It is also essential to monitor their portion sizes and avoid overfeeding. Additionally, providing them with ample opportunities for exercise, such as a spacious enclosure with room to swim and explore, can help prevent obesity in pet turtles. Regular veterinary check-ups can also help ensure that your turtle is maintaining a healthy weight.