alkalinity question / bicarb

I have a 4 X 2 X 2 (running for about a year with low fish boiload but
medium coral load and everything is doing wonderfully well). Ive been
noticing my alkalinity steadily drops over time a lot more so that my
calcium levels. Obviously water changes correct it but in the "in
between" times would dosing small amounts of Bi Carb be of any benifit
to help maintain alkalinity loss. A calcium reactor is in my wish list
...but will have to wait a little while for it.