First off, I am not very pool savvy. My pool is 30 years old. Last summer just before I closed I had some patches re-plastered and the pool acid washed. When I closed the pool it was very clean. I opened it and now I have some strange brown stains all over the pool. The water is crystal clear. The stains are not from leaves or metals(I have tested for that). I tried the Iron out test and the Chlorine tab test and neither changed the stain. I have tried brushing but nothing comes up. I also put granular on and nothing. My pool is very porous but it does feel smooth. I am at a loss! Any help would be greatly appreciated!