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 (tap water or trucked in) for filling your pool can have certain properties that can cause chemical changes as you top off your pool; especially calcium hardness & metals & minerals. 

Even if using trucked in water, have a proper chemical analysis done of that to determine what & how much balancing needs to be done. Unfortunately, many believe believe that the "pool water" that is trucked in has been treated. That just isn't so. Trucked in water can be from a variety of sources including wells, fire hydrants, ponds, streams, etc. Before adding chlorine, bromine or any oxidizer, have that water professionally tested. You'll save yourself time, money & potentially a lot of aggravation.

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.