If the water's pH is less then neutral ( 7.0 ), then correction is required to prevent the water from corroding the metal in your plumbing.
Most common "symptoms" are blue stains from dissolved copper pipes, and iron staining problems from the water dissolving any iron ( in pipes or pumps or well casings ) that is has come in contact with.
A filter tank ( usually 10" x 54" ) is filled with either Calcite or Corosex ( Magnesium Oxide ).
Both are used to "raise" the pH of water that is acidic.
Which one and how much to use depends on the pH of your water. Calcite ( which is really just cushed marble ) dissolves slowly and is used when the pH is between 6.0 - 6.8 to raise to 7.0 - 7.5.
If the starting pH of the source water is less then 6.0, then Corosex may be needed as it dissolves faster.
And it is common to use a "mix" of 50/50 Calcite and Corosex.
These are media that must be ADDED to the Filter Tank every year ( anywhere from 6 months to no more than 2 years ).
And the way to "tell" is by checking the pH before and after the filter tank.
Also, you can measure the amount of media in the tank.
Normally the tank should be "filled" to about 60% full ( and not more, or you end up with media clogging the Valve during Backwash ), and not less than 30% full.
And note, there is always 5 - 8 inches of gravel used at the bottom of the tank as an "under bedding".
Occasionally the media becomes hard, and must be dumped and refilled with "fresh" media.
Or it becomes iron "fouled" and a complete re-bedding is the best way to improve the water quality.
This type of filter tank is almost always followed by a water softener used to remove the "hardness" added by the filter tank ( as the media dissolves it increases the hardness in the water ), and the softener can help remove iron that is often in the water.
Common Tanks sizes and amount of media to use are:
9" x 48" = 13 lbs. Gravel and 1.0 cu.ft. media
10" x 54" = 16 lbs. Gravel and 1.5 cu.ft. media
And if you have nothing now - See: Complete System
Special - Tank - 10 x 54 PT1054-ADH with 1 1/4 Dome Hole DHP12