carettochelys insculpta

I have a Carettochelys insculpta fly river turtle for few months. It seem the shell has a white patch. May I know how to treat it. Eating and other activity seem normal. 1. Tank size 3ftx1.5ftx1.5ft 2. Turtle size around 4inch 3. Water parameter. 0ppm ammonia 0ppm nitrite, 0.5ppm nitrate. Tank...