need stain help bad!!

i opened my unground concrete 20000 gallon saltwater pool about 4 weeks ago, it looked like a pond. we bought the house this past winter so i am new to a pool but i understand how everything works. i shocked the pool with liquid chlorine and a few other things that i can't remember anyways the water is crystal clear and everything is balanced perfectly. i had the water tested twice now and there was no melts or algae or anything in the water. but the walls of the pool have like a tannish light brown stain on them and also in a few spots on the floor of the pool. i tried using a brush and brushing the whole pool, nothing works. the vitman c tab didn't do anything if i take a chlorine tab and scrub a spot for a few minutes the next day that spot is a little lighter thats about the only that has done something, but i obviously cant scrub my entire pool with a chlorine tab. please any help???