Use a steal wire brush to brush off the yellow stains. Also, it might help to keep the chlorine level high.
I have a fiberglass pool and I am constantly battling with iron stains. It is not impossible to remove the stains. I get brown stains in my pool and I treat them with jacks magic stain solution #1. Check out their website. They show different pictures of different types of stains. The stuff does remove the stains quickly, but it is expensive. Hope this helps.
Being a 'salt water' pool has no influence on a) how you would remove the stains or b) what caused the stains - so treat the stains as you would in any type of pool. Determine what the stains are. Algae? Metal staining? etc. Your Pool Service or local Pool Shop should be able to help determine that and also recommend the appropriate treatment.
I seriously doubt that the pool sweep is the problem here with your stains. You will have to describe the stains in order to get to the root of the problem. Are the stains: Rust color or brown? Black or gray? Green? Blue? Are the stains consistent or uniform? Are they all over the pool? Do they cover the whole pool or are they in small patches? Answer these questions and I may be able to tell you the cause. k
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