How should I shock my pool after heavy rain?

First, test your water — rain usually raises pH and lowers chlorine. If pH is above 7.6, bring it down before shocking. Then add enough shock to reach breakpoint level (10-15 ppm depending on pool size and stabilizer), using calcium hypochlorite or dichlor. Run the pump overnight, brush the next morning, and don’t swim until chlorine drops to 1-3 ppm. Chlorine shock is the way to prevent a bloom after a downpour.