Getting high calcium and low alkalinity

I would like to know what would be causing me to get consistent high calcium levels and low alkalinity levels? I stopped dosing purple up 2 months ago and my levels evened out. Now they are doing it again. I change out my GFO and carbon monthly. At the same time I also do water change. I have a 55 gallon changing 10 gallons during the change. I am a little over a couple of weeks on last water change. I noticed my co2 absorbent is spent and I am changing it out. Is that the cause or is it something else? I have 3 frags of LPS and some softies as well with an anemone.