brown stains on fiberglass pool

Hi new to this but have been fighting stains forever. I have had my well water tested and have received no indications I have metal in the water. Now the problem. I have re occuring brown stains in my fiberglass pool. This year I have tried stain free and metal free with wonderful results. The problem is the stains came back after about 8 days. I have a heater on my pool and I do use soda ash. I have read posts stating both of these may be my problem. I also read on the label of stain free that I should use a non polymer filter agent with my sand filter. What is a non polymer filter agent. I would really appreciate any help. Thanks

Have you had your pool water tested? Can you post the results? Do you have any sort of mineral system in your pool (Nature2, Frog, etc.)?

Recurring brown strains that can be removed with stain free, etc. are almost certainly metal.

Metal testing is not always reliable, for a variety of reasons. For example, if you have a softener, that would remove metals from your well water, if you collected your sample downstream of the softner.

I have a fiberglass pool with a well, and a heater. I have battled stains for the whole time I have the pool. I don't know where the metals come from, but I have successfully kept the stains away most of the time with the tricks I have learned over the years. Someone told me that some of the fiberglass pools are made with the "end of the barrel" gelcoat, and this is why they stain. I don't know if it is true, but all of the people I know that have gotten the pool from the same manufacturer in my area all get stains. Some are on city water, and I am on a well. I have a sand filter, and read up on using zeobrite, but I decided against it. I don't think it really matters. Keeping the chemistry in line is what works the best for me, and treating the stains as soon as I see them with sequestering agent, and keeping the ph on the low side. Having a good test kit is essential. The most important thing to remember is high ph along with high chlorine levels will let the metals precipitate out of solution. While the directions on the sequestering agent says to add a weekly dose, I don't. I prefer to wait till I see stains. Let me know if I can help.