It should deplete over time to a certain extent since it represents the ability of your water to buffer acids. The waste produced by your fish adds uric acid to the water which it will buffer. However other parts of your system (such as rock and sand) will naturally add back to this capacity.
How often and how much are you doing water changes? What's the bio-load? Have you introduced any new organisms lately? Certain elements of your system that contribute to the carbonate alkalinity and you may have exceeded their capacity to replenish lost capacity and need to be dosing alkalinity supplements from now on.
