After a rainstorm, the extra water in the pool and the debris dust or dirt that lands in the pool during a rainstorm will reduce the effectiveness of the chlorine. The first thing you must do is add shock to the pool water during the evening hours. Use 2 lbs. of calcium hypochlorite, 3 lbs. of lithium hypochlorite or 2 lbs. of dichlor-granular chlorine per every 10,000 gallons of water to shock the pool, or use 2 lbs. of nonchlorine shock called potassium peroxymonosulfate. Run the pool filter overnight to circulate the water so the shock clears the water.
The next morning after shocking the pool, you must test the water with the pool water testing kit. You need to check the pH, total alkalinity and calcium hardness. The ideal range for pH is 7.4 to 7.6, total alkalinity is 125 to 150 parts per million (ppm) and calcium hardness is 200 ppm. If using the water tube kit or test strips to test the water, compare the water or strip color to the color chart with the kit to see if any of these levels are high or low.
Adjusting pH and Alkalinity Levels
If the pH level in the pool is below 7.4 ppm, use 6 oz. of soda ash per every 10,000 gallons of water to raise the levels 0.2 ppm. For pH levels that are above 7.6, use 12 oz. of muriatic acid for every 10,000 gallons of water to lower the pH 0.2 ppm. Soda ash will raise the total alkalinity, and muriatic acid will lower the total alkalinity. When the total alkalinity and pH are not within range, the other pool chemicals are not stable and will not work the way they should.
Adjusting Calcium Hardness
Calcium levels in the pool are usually not a problem, but when rains come and the pH and chlorine levels are off, the calcium hardness may be too high or low. If the calcium hardness is lower than 200 ppm, use 1 1/4 lbs. of calcium chloride per every 10,000 gallons of water to raise the levels 10 ppm. For lowering levels over 200 ppm, use 1 lb. of anhydrous trisodium phosphates per every 10,000 gallons of water to lower the calcium hardness by 10 ppm.