New pool owner needing help. 13,000 gallon gunnite saltwater pool, filled with city water. I'm right at a year of pool ownership and have always had crystal clear water. Parameters are typically spot on but anytime there's been a minor issue, it was low chlorine or high PH and I would remedy that. Not sure if this was the cause, but I shocked the pool a couple months back and within a day or two I noticed some brown streaky stains. I tried to rub some Vitamin C on a stain on the swim deck, to no avail (rubbed a couple tablets pinched between fingers on stain till tablet was gone). Would that have been enough time? I've also tried rubbing a chlorine tablet on it... That did nothing at all. At worst, PH was arguably a little high (8.0) when I shocked it. Any advice? Thank you in advance!