I have no doubts that your stains are caused by iron. Ascorbic acid would not remove brown copper stains at all!
Still get a recomfirmation on the copper levels in your water. For this you can use a copper test strip or even an aquarium copper test as long as it will register in the range of 0 to 1 ppm. If your copper is 1 ppm or above bit the bullet and drain.