I found black dots on my turtle’s head and one foot; what is it, is it serious, how to treat?

Even black dots on the turtle’s skin are usually melanin deposits, a normal phenomenon like freckles on human skin, and generally not serious.

But if the spot smells foul, it may be fungal or bacterial infection from poor or wrongly changed water. Change the water regularly and disinfect the pool to avoid microbes from the turtle’s metabolism.