there is a correlation between Ca and ALK.. but not directly.. you can deplete one or the other quicker depending on the rest of your tank chemistry,, for instance, magnesium plays a big role in being able to maintain your Alk levels. if you are absolutely religious about water changes then Mg should remain where you need it, and if you have fish and a few softies, water changes alone might work, but if you have LPS or SPS your rate of uptake will increase as the corals grow and you will need someway to measure this change,, you will likely need to make small adjustments to one or the other, so you will need both test kits. its a small price to pay to avoid an expensive mistake