Jeremy:
Man does that irritate me. I long post lost to cyberspace with a new request to log in.....
OK, I'll try again.
I see people every day that have problems keeping the alk up when the calcium is ok using the 2 parts.. it's actually quite common for the alk to be depleted faster than the calcium from these supplements.
True, but it is mostly mathematical illusion.
If you start with 420 ppm calcium and 4 meq/L alkalinity, and then let your favorite coral calcify to reduce the alkalinity to 3 meq/L, how low do you suppose the calcium will go?
It actually will only drop to 400 ppm, which is within the noise of most test kits. Many reefers seeing that say 'Whoa, my alkalinity dropped by 25%, but clacium hardly changed. I'd better add buffer...."
Another illusory reason for alkalinity declines is that most reefkeepers strive to keep calcium at about natural levels (say, 400 ppm or so) but want to push alkalinity far above natural levels (say, 4 meq/L instead of 2.5 meq/L). Since at least some of the salt mixes better approximate the natural alk levels than the elevated alk levels, any water changes will have the appearance of a decline in alkalinity.
There are two potentially significant reasons for alkalinity to decline independent of calcium.
One is the nitrogen cycle. The conversion of ammonia to nitrate creates substantial acid that reduces alkalinity. If you remove that nitrate with water changes, over time you will notice the effect on alkalinity. If, however, the nitrate is used as a nutrient by organsims (like algae) or is converted to N2 in a sand bed, you get that alkalinity back.
A second true alkalinity drop comes when magnesium and strontium are incorporated in to growing crystals of CaCO3. In this case, carbonate is used, reducing alkalinity, but calcium is unchanged.
I'll point out, however, that many tanks run for years on perfectly balanced limewater and do not show alk/calcium imbalances of significance.