are you adding ballanced amounts of calcium and carbonate? just adding calcium can cause low alkalinity. are you doing enough water changes? that can also have an effect. just a couple of ideas. i forgot to ask is your tank fresh or salt?
my tank is fresh and i did a water change just on monday, i dont add any calsium or carbonate to my water...btw is carbonate just carbon because ofcourse i have carbon in my filter.
the calcium and carbonate is to promote reef growth so its no concern to you, is your carbon filter new / still good? Is your filtration unit big enough for your tank?