Based on these readings, I would start by adjusting the calcium level. The calcium level needs to be at least 200 ppm, but I want to increase it to at least 300 ppm. The reason I want to increase it to 300 ppm is because throughout the pool season evaporation, backwashing and "splash-out" will lower the calcium levels. This goes for all chemicals. By increasing it to 300 ppm (or even 400 ppm for that matter), the calcium level will still be within an acceptable range in the event I need to add fresh water to the pool. To calculate how much calcium hardness you will need, you simply subtract: (300-120= 180). I need to increase the calcium by 180 ppm. You will typically need 1 1/4 lbs of calcium chloride to increase the calcium level by 10 ppm in a 10,000 gallon pool. Multiply the 1 1/4 lbs by 2 (for 20,000 gallons) and you get 2.5 lbs needed to increase the calcium level by 10 ppm per 20,000 gallons. I will need this dose 18 times (10 ppm x 18= 180 ppm). 18 x 2.5 lbs = 45 lbs. Now that I know the amount I will need, I can start adjusting my level. Calcium chloride can be a harsh chemical if not handled properly. Do not add more than 10 lbs at a time per 10,000 gallons of water. For this pool, I will start by adding 15 lbs (20 would be o.k. since it's 20,000 gallons) and retest then add the second dose 12 hours later.