Bar soda (Schweppe's, about a tablespoon or two per 50 gallons) and don't panic---the ph self-adjusts very rapidly on its own.
Of all chemical accidents you can have, kalk is one of the most benign. The tank will mostly suffer from dust all over everything, and a turkey baster can be used to puff the dust off the corals. It'll likely be ok. I've done it several times.