Good question. What's in the tank? Fish, corals, etc...also how much light do you have? What I'm trying to get at is, how quickly will calcium get used up in yur tank?

Assuming your inhabitants can survive such low alk (if you're talking about 2 meq, not 2 dkh, it's not super low) and assuming that your tank will consume calcium through photosynthesis (good light, photosynthetic animals, coralline algae) then I think the best approach would be to simply add buffer; I'd use plain old baking soda unless your ph starts dropping. I'd gradually bring up the alk with the buffer over a few weeks' time while the calcium is dropping, and you shouldn't get a snow storm, and you'll balance out in a while.

I'd consider marine environment salt in the future, at least I think that's what it's called. I've never used oceanic, but a good thing about the marine environment is that it's pretty well balanced out of the bag.