That is fishtail.
Well water with too much iron cancels out copper.
Getting a water filter system such as culligan water treatment system, they put in, what is called iron out, which will flush the iron out.
Giving the right dosage of copper bolus by weight is needed and if no improvement, you can follow up 4 months later until the hair is growing back on the tail head. Look closely at the tip, is it bald or has little hairs there?