Dark stains on vinyl liner

Hi, We have a 30000 gal inground pool with a vinyl liner, I've been having problems with dark brown spots that disappear when I shock, but come back after a day or two. The do not respond to brushing. A friend is suggesting that there may be a bit of water under the liner producing mildew. I've been told to shock with 6-8 gals of liquid chlorine, has anyone experienced this ? Any suggestions greatly appreciated.
My other measures seem OK, ALk and PH tend to be low/normal in my pool and my pool gets full sun, is crystal clear.

Thanks!!

6-8 gallons of chlorine may or may not be okay in a vinyl pool--it depends on your CYA level. Also, you say that alk and pH tend to be low/normal, but those are relative measurements--do you have any metals in your water? Copper/iron?

Janet

I still need some help. i just ordered the test kit, but using what I have I get the following: TC 10 FC 10 PH 6.5 ALK 40 CYA 0-10.
I added 5 lbs CYA about 3 weeks ago and my reading did not budge.
I shocked last night so the chlorine #s are not usual. My stains are not metallic. they respond to shock, but do not respond to the brush. I did not have them last year. I need to know how many PPM of bleach I need to get to for a strong shock. I have 6% Ultra from WALMART, but I need to know how much to use.
Any help is appreciated.

Nick

We had this last year, with the same results, that it responded to shocking and then came back. But this year I switched to the BBB method, and our pool water looks like liquid glass, and no stains!