I have a 22k gallon gunite pool with a SWC. Within the last month, a yellow tint has appeared at the bottom of the deep end. It is only in the deep end (8 ft), and only on the bottom of the pool, not the walls. There is a pretty clear line where the tint stops when the bottom of the pool starts to rise to meet the wall. I shocked the pool and it had no effect. I tried the Vitamin C test and it had no effect (earlier in the year we had a metal stain problem and the ascorbic acid worked like a charm). I've tried brushing and that didn't do anything either. Any ideas? I don't have current chlor, ph, etc. since we just shocked yesterday, but the chlorine is typically around 3 and ph is 7.2 to 7.4.

Thanks in advance.