Light brown stain below water line

Please Help!! I have an under ground pool, that holds 18,000 gallons. The pool liner(vinyl) and steps were white and blue when the pool was opened this spring. About a week ago every surface under the water became stained a brownish/orange color. I have tried brushing without any success.
My test results using the PS233 test kit are:

FC 19
CC .5
TC Dark orange
pH less than 6.8
Alk 0(test tube water turn red without adding any R-0009)
Cal 200
CYA Greater than 100

I shocked the pool 24 hours before getting the above results. I'm currently using chlorine tablets, guess I should have just gone with good old bleach.

Before shocking the pool yesterday I noticed that the stain seem to be disappering in small spots all over the pool. I brushed around the clean spots thinking it would remove the stain, but I was unsuccessful.

Tonight I added 3 lbs of baking soda to try to raise the Alk. SHould I go ahead and add Borax to raise the pH?I guess I will need to replace some the water to lower the CYA, right?

Any Help is greatly appreciated.

Sounds like you may have some metal staining. You can try crushing some vitamin C tablets and put them in a sock , or some ph down in a sock, and rub it on the stain. If the stain lifts, it is mineral. If you rub a chlorine tab on the stain and stain lifts, it is an organic stain. With a cya over 100, I would drain the pool down 1/3 and add fresh water, do this a couple of time till you get your cya down to about 50 check out this chart:

Use the info in this chart to help you figure out what levels of chlorine you need to maintain in your pool based on the amount of CYA (cyanuric acid, also called stabilizer) that you have in your pool. (FC = free chlorine)

Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
=> 60 - 90 ppm . . . . 5 ppm . . . . . 10 ppm . . .. 20 ppm
=> 100 - 200 ppm . . . 8 ppm . . . . . 15 ppm . . .. 25 ppm

You can add washing soda, which will take the alkalinity and ph up. You need to get them in range so that the the water is not so acidic, which can damage a vinyl liner. After you see what the stain is (organic or mineral) we can help you more. Don't be discouraged:)