Dark stains on vinyl liner

[nisqu]

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!!

[aylad]

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

[nisqu]

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

[buckeyemom]

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!