See this post for an example of adding oxalic acid to a pool with calcium in it. It results in calcium oxalate powder. It also apparently can interfere with the CYA test causing a falsely higher reading.Katydid said:I cleaned steps and ladder with united chem pool stain treat (oxalic acid, 5 lbs) and it worked. Didn't have time to do the rest of pool so hired pool company to "clean my pool". They dumped 6 lbs of oxalic acid and left me a bigger mess. I had a heavy white residue in the bottom of my pool which took me forever to filter out.
While oxalic acid is a reducing agent and also lowers the pH, we recommend using ascorbic acid even though it is more expensive because it does not have the precipitating side effects. You can buy ascorbic acid at PC Networx for $7.50 per pound for 5 pounds (it's also at The Chemistry Store for $11 per pound for 9 pounds so a lot more expensive). Note that the chlorine will react with the ascorbic acid so to avoid wasting money you'll want to get your chlorine level down towards zero first. To help prevent algae growth, you can add Polyquat 60 where if you add a full start-up dose it should lower your chlorine level as well. After the ascorbic acid has removed the stains you can replace some of the water to lower the CYA level and before you refill you would use a metal sequestrant if the fill water has metals. You only slowly bring your pH back up after you've added the metal sequestrant. See Metals in the Water and Metal Stains for more details.