I couldn't help but notice your profile says your in SE Texas so that's probably why the pool guy thought you had metals in your water. Have you ever tested for metals? If you do have metals you might use one of the other Jacks products, like the pink one made for metal staining.
Our Austin location has a world of a time trying to fight staining, she swears it was the salt system causing it. Now that she's removed her SWG completely and it's still staining I wonder what she'll blame it on.
I'm not aware of a test you could perform yourself but that's not to say one doesn't exist.
I've had a private lab, ANAlab do some testing for me. They can test for just about anything you can dream up that could be in water but we just do routine coliform testing as sort of an insurance against parasites.
We've never actually tested positive for any but it just gives us something to show to anyone concerned about swimming in our pools.
I had our Austin pool tested and sure enough, as suspected there's metals in the water. I'm sure you could probably find metal in any water, maybe the levels are just higher in some areas. Central and West Texas being some of the areas for sure.
there are test kits available for iron, copper, and manganese. Check the Taylor and LaMotte websites. There are also test strips available. Check the AquaChek website. The TF100 and Taylor K-1005 include a test for the metal calcium. Light I said before, the purple stuff is often recommened to help prevent scaling of the salt cell from calcium and also to help prevent scaling with new plaster since it will also sequester calcium to some extent.
Personally, I don't recommend it unless there is new plaster, a known metal problem, or calcium levels are high. It really depends on your water balance numbers. If you have very high calcium then it's probably a good idea.