Often we have protracted periods of no rain here in Louisiana and then day after day of heavy rains. During the rainless times, we usually have to add water because of evaporation, so that means we have to watch chlorine levels in the pond.
Then one day our hard rains will start. The pond fills and overflows. The overflowing is OK, but the rain is cleaning filthy air, air filled with the pollutants we put in it from our exhaust pipes, airplanes, chimneys and factories. All of that chemical waste is washed from the air onto our soil and into our ponds.
After the first rainfall following a dry time, watch your pond carefully. If your fish come to the top trying to breathe or your plants begin to yellow, change at least half your water. Don't forget to put dechlor in the new water.