that is what I thought you meant. I LOVE THAT AQUARIUM. I have been there many times and plan to go there again when I go down to San Diego for Christmas. I really like their touch pool outside in the back where you can play with the starfish. I am also in awe of that HUGE indoor display tank with the girabaldi (wow did I massacre that word) and leopard sharks. Anyway...sea water is naturally lower than what we are trying to do with our tanks. our levels are like steriods for corals. If we did it with just sea water, we would experience about 1/4 of an inch of SPS growth in a year. No thanks for me. If I were you, as much as you like natural sea water, I would switch to a good sea salt mixture with RO water. RO tends to be free at a good LFS, although in So Cal, not much is free. You will have more success with your tank that way.
Seachem makes the best calcium additive IMHO. It is the blue bottle of powder. If you switch to Oceanic (which i use currently) or Reef Crystals (which I buy when it is on sale) and dose just a touch weekly, you will have no problems maintaining high levels of calcium and DKh.
As for magnesium, I have never even thought of dosing that. Iodine, magnesium, strontium and all the other trace elements are in the supply you need with a good sea salt mixture, another benefit to making your own. Just my :twocents:
-Doc