Hello all, I am having some trouble I was hoping you can help with. I have a underground pool with liner. About a week ago, I noticed black spots in numerous locations of the pool wall and floor. I scrubbed and added 4 lbs of shock (20k gal pool). Two days later, black spots came back, chlorine level was zero. (I'm using the type of kit that you add drops to a little sample). I added 5 lbs of shock and vacuumed again. Yesterday, black spots came back, chlorine level again zero. I added 5 lbs again and vacuumed. Today, black spots not back but chlorine is gone. A just ordered black algae treatment. So, is the black algae eating my chlorine? What should I do? Thanks a million.