[Delphinus]

Oh, I'm going to hate myself for not keeping my mouth shut ...

But, Brad, if tap water measures out at, say 270, doesn't the salt mix itself also contribute, so if the freshwater is 270, and there is (say for example, since I don't know the #'s offhand) 80 ppm in the salt, won't the makeup water in fact be 350?

I always maintained a Ca value of 350, and an alk of 2.7 meq/L using freshwater with no additives, on my first reef tank (which is softies [mostly mushrooms actually], my ritteri anemone, fish and snails. Not really a high-calcium draw, except for my one purple acropora, which for some reason really excelled in those conditions. No other SPS really took off like that, which is what I would expect with numbers like that).

Of course, if maintaining an alk of 4.0, which would be necessary once there are more stoneys involved, and that calcium will drop pretty quickly, so some means of replenishing (beyond makeup water, or water changes) will be required.

As for "most economical" way .... Unfortunately I think that's a rather loaded question! It totally depends on what you're keeping, and what are your goals. For some, "adding nothing" will be the most economical for some; for others, kalk; for others, two-part additives such as Seachem Reef-Builder and Reef Calcium (or B-Ionic 2-parts), for yet others, calcium reactor.

I found with about 16 SPS frags in my other tank, the calcium and alkalinity draw had me dumping in so much additives that I figure my calcium reactor will pay for itself within two years. That's with frags, so I can only imagine there will be a lot higher draw once they start to form larger colonies.