Mustard stains but not algae, what is it?!

[chem geek]

Try the ascorbic acid (vitamin C) test and if the stain fades then do that treatment described in the link I gave earlier. You likely have an iron stain in spite of their not being any such measurement in the water -- all of the iron is in the stain and may have precipitated out completely so as not to measure any metal ions left in the water.

[Sunburst]

We're having the EXACT same issue with our vinyl iner inground pool. Large brownish stains over half of the pool floor. The stains however definitely feel slimy, but you can not brush away the stain no matter how hard you try.

I'm going to try the vitamin C trick tonight to see if this has any effect. Please let me know if you find a solution.

[summerlover]

we have the same thing and have done the acsorbic acid twice at $48 a time and it lasts less than a week. Our water tested negative for iron, 1st time pH and alkalinity were low and the pool store told me to use 17 pounds of soda ash, after adding only 10 pounds it shot my levels way up and made the water cloudy. Now after the 2nd time I have no chlorine and my phosphates went from 100 to 500. The water is crystal clear but with the staining looks like H*@L, I am also at a loss and the people at the pool store don't know what to tell you. Do I try phos free or algicide? Initially I was told I wouldn't need to keep doing the stain free more than once. I have scrubbed the pool and nothing helps

[chem geek]

The ascorbic acid treatment only removes the stain, but you need a metal sequestrant to keep the metal in solution and that needs to be added regularly. The better metal sequestrants use HEDP and don't break down quickly by chlorine. Read Metals in the Water and Metal Stains for more info. If you don't use enough metal sequestrant (or use a poor one) or you raise the chlorine or pH levels up too high or too quickly, then the stains can return.

There are other techniques to forcibly precipitate metals so they can be filtered out with a good filter or with batting material, but these techniques (which mostly involve raising the pH) can deposit stains back onto pool surfaces. This works OK for vinyl where the staining is less likely, but is not a great technique for plaster pools or fiberglass that stains more readily.