Mg will be released if it was originally a component of the media being dissolved. Whether this would be in an ionically availble species, not sure.

There will certainly be some dissolution of the substrate within our systems, but as I stated, not in any amount I would take into consideration. My understanding is that a pH of 7.7 is required to disolve Ca. My tank doesn't usually get that low. If things made of Ca started dissolving in my tank, I'd be worried about my Ca based inverts.
Now, in a 150g fully stocked SPS tank, I used about 20 pounds of reactor media in addition to kalk top off to maintain Ca and Alk per year. This tells me that anything comeing from the substrate is at best negligible.

Calcium reactors require pH 6.4 and 6.8 for proper release of Ca from substrate in the reactor. The reef water apparently has a higher buffering capacity so the low pH does not affect the tank pH

Yes, a reactor works best between 6.5 & 6.7. Ca will start to dissolve though in the high 7's. The low pH in a reactor is dripped back into the tank at such a slow rate that off gassing of the CO2 is quick and the high Alk will cover any other difference. Mainly though just dripping the effluent into the system flow will remove any CO2.

Here is a great link to Ca reactors. Follow the links at the bottom for articles expanding on the chemistry by Craig Bingman and Randy Holmes-Farley.

http://www.reefkeeping.com/issues/20...ture/index.php

Know in the 3 years I had a DSB it's level did drop and would need to add more sand occasionally. Some of it, even with sand sifters was probably compaction, but some of it must of dissolved.

Now did I rely of even think of the sand to maintain my waters parameters, no, for that was water changes.

could be the sand/minerals dissolve bit by bit normally, but not optimally in your average reef aquarium

Again, that's why some rely on a reactor.

lol yea ok tks guys, i think I've got my answer