pH and Your Water
pH is the term we use to indicate if something is acidic or basic. The scale for pH is 0 to 14 with 7 in the middle being neutral. A number below 7 is acidic and above 7 is basic. Drinking water is ideally between 6.5 and 8.5. It can be above or below those numbers and not be a health concern because our bodies maintain a pH equilirium, but once it starts getting too acidic or basic other problems can occur.
Low pH
If the pH is low (acidic) it is not harmful to drink. However it can start leaching metals and other materials from your roof and catchment system that could be harmful or cause aesthetic issues. For example copper leaching from copper pipes can stain sinks bluish-green. Acidic water could leach out toxic metals into your water that could be harmful, like lead from lead nails and old lead based paints. Other anadotal concerns over low pH water is concerns for tooth enamel softening, but this has not been confirmed by research. You water can take on a metalic taste. Your soap won't lather as well.
High pH
If the pH is very high (basic) it is not harmful but it can make the water taste bad. Very basic water is usually filled with minerals and the water is called "Hard". It can make stains on clothing, dishware, create deposits on pipes and fixtures and even make your hair look dull. Also coffee will taste bitter. Scale build-up can be a problem in your pipes and the efficiency of electric water heaters can be reduced because of the scale build-up.
Testing for pH
You can use simple pH test strips or any kind of pH testing device as long as it covers the scale that you need. In other words if you think your water may be very acidic and the pH test you buy only reads down to 6.0, then you need to find a tester with a lower cale.
Raising the pH of your tank water
Rain is usually acidic so unless you have a concrete water tank (The calcium in concrete makes the water basic) your harvested rainwater will probably be acidic. Baking Soda (Sodium bicarbonate) is commonly used to raise pH. It will add a little salt (sodium) to your water so if you are on a sodium free diet this may not be a good solution for you. Calcium carbonate also raises the pH and is most effective in granulated form (large surface area). It will add Calcium to your water which increases hardness.
Lowering the pH of your tank water
Basic water is usually treated with a water softener that uses an ion exchange process to remove minerals from the water. These systems usually require a specialied mixing tank and periodic refreshing. They are available commercially. It is a complicated process not easily accomplished without a background in this type of chemical process. Normally rainwater isn't basic (hard) but if it is stored in a new concrete tank minerals from the concrete can leach into the water and make it basic. This condition usually stabilizes as the tank ages.