The relationship between calcium, carbonate and pH in the tank show that if you affect one of the parameters, you may affect the others.
Let's take your questions one at a time. Calcium:
normal salt water may have a high calcium concentration. It is in much higher concentration that the carbonates and is excessive in the relationship identified above. It is also easier to get calcium in the water than it is carbonates. All hard corals and especially SPS
need calcium to build their skeletons. If you don't have a lot of SPS
corals actively growing, your calcium levels will not go down much. Carbonates (alkalinity):
Not found in excess in tank water. Carbonates are required to complex with calcium to form the skeletons of CaCO3. Typically, carbonates are the limiting factor in the relationship identified above. This means that if ALL the carbonates were to suddenly bind to calcium in the tank and precipitate out, calcium would still be available in the water. pH:
Adding carbonates (baking soda) to the tank liberates CO2 that causes the pH to go slightly acidic. If this is a problem, put 2 1/4 cups of baking soda on a sheet pan and bake it in the oven for 1 hour @ 300F. This drives off the CO2 and any water that is present. When finished, dissolve the baking soda (now soda wash) in one gallon of warmed water and use. This solution will allow your pH to remain higher when you add the carbonates. Other reasons for a lower pH include a tight house that does not allow for CO2 to dissipate.