This is a salt water pool with a propane heater. Black stains are spreading into the swallow end. Pool guy put muriatic acid in the pool, didn’t help. What else can be done. I scrub with a brush and it doesn’t budge?
We need to know a lot more about your pool with pictures and descriptions of the equipment, how you take care of your pool and how you are testing your water. There’s too little information to give any decent advice.
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