First time poster here. I'm in the process of getting my pool back in shape for spring/summer and am having some problems with some stains (see pics). It's approximately a 10,000 gallon salt water, inground, pebbletec pool. The problem I have are these light brown/tan stains that look like sand or or dirt. I've tried chlorine tabs and vitamin c tablets on the stains, in various spots and for various lengths of time, and there have been no changes. I also purchased the stain removal tool where you put acid in the container and let it sit on the stain, and that made no difference either. Based on my research, I'm guessing this is a scaling issue? If it was, though, I would have thought the muriatic acid applied via the stain removal tool would have worked. So, to say the least, I'm a bit perplexed. The pics are from the spa that show the issue I have throughout the pool. Thoughts?