Sometimes, either as a result of accidental over chlorination, it is necessary to lower the chlorine level of pool water. This is achieved by treatment with either Sodium Thiosulphate or Hydrogen Peroxide. As an example for each 1 p.p.m. (mg.l)chlorine in 45,000 litres there will be 450gm chlorine. For each gram of chlorine add 2gm Sodium Thiosulphate or 1.5gm 30% Hydrogen Peroxide.