The only additive I was/am using is B-ionic which is NaHCO3 (baking soda) based.  I've not noticed a pH raising or lowering effect from it.  I do (at least I think I do) have a CO2 degassing problem, which causes a lower then expected pH.  But even before dosing b-ionic (early in the tank's life), I noticed my alkalinity and calcium slowly dropping to levels of concern (below 2 meq/l).  That is what got me started on B-ionic.

Jon,

I would have to argue that because your tank had been conditioned to using additives, a “cold-turkey” stop of adding them could make the pH drop much faster then in a tank that has never used additives and much fast then the aragonite could have buffered the water from the pH drop.

Ok, so I'm stranger then the average WMAS fish freak®

After the discusions a few weeks ago regarding aragonite solubility and the fact that I was interested in calculating my tank's calcification rate, I stopped all chemical additions to my tank, but kept measuring the water parameters.  In one week, my alkalinity dropped from 3.5 meq/l to 1.5 meq/l.  I stopped the experiment here as my green/blue acropora started looking pale.  Amazingly, my calcium dropped from 400 ppm to 360 ppm (exactly what the chemistry would predict!).  My pH never got above 8.0 or below 7.7 during this experiment.

What this shows, at least to me, is that my aragonite substrate and live rock did not do anything to "buffer" my water at pH=8.2.  I can't say for sure that none dissolved, as it would be like adding a balanced additive to the tank and would be impossible to measure, but my alkalinity decline was very steady suggesting to me that nothing adding to it.

Now to the good part... based on the data, my tank has a calcification rate of 2 kg/m2 per yr.  This is kinda on the low side, if you recall the average reef calcification rate was given as 4 kg/m2/yr and Craig Bingham has suggested that normal reef tank calcification rates range from 2 - 20 kg/m2/yr.  My tank has a mixed group of corals.  I've got 3 small sps, a frogspawn, fungia plate, very small clam, maybe 1 square foot of star polyps, and 4 -5 different soft corals in 20 gallons.  The tank seems crowded to me and I was expecting a higher calcification rate, but numbers don't lie (do they?).

If you get a wild hair, stop additives and measure the decline in alkalinity (it may not take a full week if your calcification rate is higher then mine).  Post the results here, I'd be interested to see what other tanks are doing.