Yes, yes, I know. There's plenty on here already about phosphates and I'm pretty sure I know the answer to this but I needed to ask anyway:
This was my first year with a pool and a very well-meaning friend of mine who has had a pool for much longer told me to learn from his mistake and be sure to check phosphate levels.
He states that a few years ago 'pock marks' (small holes) starting appearing in his concrete and after months of pool stores not knowing the answer, they finally realized his phosphate levels were too high and once he lowered them he stopped getting new pock marks but the damage is already done with the old ones.
There are several things that don't make sense to me about this - least of all how high phosphates can eat away concrete but I thought I'd throw it out there just in case I missed something in my research.
I of course immediately told him to start testing his water himself, not to rely on pool stores 'knowledge', and referred him to this website!