Over 14 gpg   Extremely Hard 10 to 14 gpg   Very Hard 7 to 10 gpg   Hard 3 to 7 gpg   Moderately Hard less than 3 gpg   Slightly Hard gpg = "grains per gallon" 1 gpg ≈ 17 ppm

Remember that the source water for filling your pool can have certain properties that can cause chemical changes as you top off your pool; especially calcium hardness & metals & minerals. 

If your tap water is "soft" or "hard", those chemical make ups, make a difference.  The use of water softeners makes a difference.  PLEASE tell us this information when you contact us so that we can better help you.