To keep a pet turtle healthy, you should ensure that they have a suitable habitat with proper lighting, temperature, and water quality. Provide a balanced diet that includes commercial turtle food, leafy greens, and occasional treats like insects or fish. Regularly clean their enclosure, monitor their behavior for any signs of illness, and take them to a reptile veterinarian for check-ups. Additionally, provide ample opportunities for basking and swimming to support their physical and mental well-being.