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!!