I have a yellow stain as well and i know what caused it.
Yellow stains in pool.
I used an algaecide and the next day there was a yellow stain on the wall from my return line all the way around to my skimmer.
Pool steps can leave stains and rust on the finish and age can take a toll on the finish.
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I am stuck with two issues.
Unlike algae iron deposits resist scrubbing and take a lot of elbow grease to remove.
Sprinkle approximately 1 2 bag of suncoast stop stains over the same area.
Sometimes it can be fairly easy to get rid of stains but other times those hard to remove spots can take a bit more than a simple scrub.
So it only ran for 3 or 4 hours which may have helped to cause the problem.
I also forgot to switch the pump from auto to manual.
Your first clue to diagnosing a pool stain is to check out it s color.
Shades of yellow can be a sign of excessive iron.
Iron can be introduced into a pool by well water or pipe corrosion somewhere in the pool system.
When there is too much of a mineral present in the water it can begin to stain the pool.
It looks a lot like it would if you had mustard algae but when you brush it doesn t come off.
Pool stains on the wall or floor of a swimming pool can be caused by metals in the pool water or by an organic material left in the pool.
The most common blemish however is usually fading caused by sun damage and tears caused by equipment or debris in the pool.
I have read some other posts pertaining to this but still a bit confused on what to do.
If a yellow or reddish color occurs it is an indication that the stain is copper.