I have 2 pools that had high FC, and recalled reading about hydrogen peroxide to reduce FC. Normally I would just turn off the chlorinator and let the FC come down naturally but the water is cold, there's not much sun around and I found some cheap hydrogen peroxide by chance and thought I would share my results.
From a quick search I found this quote;
"27% hydrogen peroxide at a reasonable price of $17 per gallon. It would only take 115 fluid ounces (around 3-1/2 quarts so somewhat less than one gallon) to lower the chlorine by 50 ppm in 10,000 gallons."
With that as a starting point and using Google to convert to metric and then adjusting for my pool I get; 1,040ml of 27% H2O2 in 29,000L to lower FC by 20ppm.
As luck had it I found 5 litres of 190g/L or 19%w/v hydrogen peroxide for $10, a cancelled order. Adjusting for 19% I get; 1,540ml of 19% H2O2 in 29,000L to lower FC by 20ppm.
I don't know where I went wrong but my observed results are out by a factor of 2.
Day 1: FC = 25. + 1,500ml 19%H2O2 + 1.5hrs Cl2 run time + 24hrs. FC = 15.
Day 2: FC = 15. + 1,500ml 19%H2O2 + 1.5hrs Cl2 run time + 24hrs. FC = 5.
In reality it was 1.5L of 19% H2O2 in 29,000L to lower FC by 10ppm.
Or, 530ml of 19% H2O2 in 10,000L to lower FC by 10ppm.
Or 68 fluid ounces of 19% H2O2 in 10,000 gallons to lower FC by 10ppm.