We have a 35,000 gallon, in-ground vinyl pool. Last week, water was beautiful, all chemicals perfect, then black spots became evident in the deep end of pool. These spots aren't huge, but they don't respond to brushing at all. Went to pool store ---they said it was probably black algae. Bought algaecide, followed directions, upped FC, brushed, waited. Zero results. Did entire process again---algaecide, shock, brush, backwash, etc. still no results! Any ideas? We are leaving town for two weeks and I'm terrified of returning to a "black hole"!!!