Soft water just removes calcium hardness. It has nothing to do with any possible dissoved metals which can come from numerous sources such as pipes, the ground your water source comes from, pollution, etc. It is possible to have soft water with a high iron content. First step would be to check your water for all metals. I have a big distrust of pool store testing from my own experiences. One time I brought in two different samples, one from my spa and one from my pool, and did not tell them that they were a connected system and that I had my spillover running for 4 hours right before I took the samples. One sample tested as having very low CYA and the other sample was low on CH and had very high TDS. I suspect it was error on the part of the (what looked like) high school girl who was doing the testing. There is a compnay that is supposed to make a metal stain identifying kit (I think it's Jack's Magic but I don't know anything about it) and Taylor sells kits for testing for varous metals as do other testkit companies (LaMotte). There might even be test strips that will at least tell you if metals are present but I would not trust them to tell you at what concentration. Iron is your most likely culprit and you probably still have a lot of copper in your pool. It might not be in the water but it is deposited on the surface and can leach back into the water if your pH drops.
Good luck on fighting the stains!
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