so why did my alk drop when i used the pro-MARC?
No idea son.
Alk can be fixed - if it has a much higher Ca, Mg and Sr as is indicated I think that it worth the effort of dosing with buffer.
I have read conflicting reports, here for example, a chap finds the Alk at 10.8 dKH
Report on Tropic Marin Pro Reef Salt - Page 2 - The Reef Tank but there is also a chap that says it was low - 2 dKH but without proving it. the Ca is very high.....
Another one says that its at 8dKH
The most definitive answer I have found so far is here:
Tropic Marin VS. Tropic Marin Pro Reef?? - Reef Sanctuary
Note the chaps says the following - seems that the Alk is low because it is aimed at folks that have a high Alk, BUT Lanzo he also notes that it will not bring down your alk.
"Tropic Marin at a specific gravity of 1.024 has:
* 9-10 dKH
* 380 ppm Ca
* 1100 ppm Mg
Tropic Marin Pro Reef at a specific gravity of 1.024 has:
* 6.5 – 7.0 dKH
* 420 ppm Ca
* 1350 ppm Mg
Tropic Marin Pro Reef was designed for reefs that tend to acquire a high alkalinity after years of using calcium reactors and kalkwasser. Keeping a lower alkalinity has been found to allow corals to better absorb calcium. Depending on the values of alkalinity in your reef , it might be best to use Tropic Marin instead of TM Pro Reef, or vice versa.
In slightly different terms, the ONLY good reason for using Tropic Marin Pro-Reef is if the alkalinity in your tank is continuing to rise to an undesirable level and you don’t want to contribute to that rising level with water changes.
It is important to note that using Pro-Reef will not lower your alk but will not contribute to the rise as the original formula might. That slightly higher levels of Mg and Ca in the Pro-Reef are NOT a reason for using it because you are adding so much Ca and Mg on a daily basis anyway to your tank."
Pete, this is pretty cool as he lays out why you should/shouldn't do Pro reef, for the addition R70 you pay extra for the Pro Reef and a 40ppm rise in Ca I am reconsidering the salt and will probably go back to TM standard . Also check the Tropic Marin website, has a good table that explains which you should use.
Tropic Marin