Sounds like you have iron in your fill water. Do you have a well? If so, you may always have iron OR you may have a problem with iron seasonally.
Regardless, stains that are removed by ascorbic acid (vitamin C) are metal based, and usually iron. One IMPORTANT point: ascorbic acid removes stains, by dissolving the metals and putting them BACK into your pool water, but only temporarily. Once the chlorine destroys the ascorbic acid (which happens pretty much as soon as you have measurable chlorine levels) the stains tend to come back, UNLESS you've done something to remove the iron from the water while it was dissolved.
I gather you also have had some slimy deposits, leading to the suspicion of algae or bacterial slime?
Do this:
1. Use bleach to maintain adequate chlorine levels, by dosing each evening as needed.
2. Order a K2006 testkit from Amazon (see link in my blue signature block). When it arrives test BOTH your pool water AND your fill water. (Do NOT test fill water for CYA -- it has none, and the K2006 only has enough reagent to test CYA a few times.)
3. Meanwhile, find a CLEAN white 5 gallon bucket with a lid. Fill it with 4 gallons of water, from what ever source you filled your pool water. Add 1/2 cup of baking soda, and 1/4 cup of bleach. Mix and cover. Let it stand for 2 days, and then check. If you have iron or other metals in your fill water, they will probably show up as colored debris or dust at the bottom of the bucket.
4. Also, brush your pool carefully, every 2 days. If you chlorinate AND brush, that should eliminate any remaining slime.
5. Once you have test results (K2006 + bucket) post that info, along with a description of current water conditions AND info on what pump and filter you have (model numbers).
Good luck.
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