There is another brand of Calcium blocks from Sea Labs. They are manufactured and marketed for saltwater aquariums. I have used them for over 10 years on multiple tanks...from 2 gallons up to 600 gallons. They don't add up to be the most cost effective way to dose calcium, but they are the simplest thing i've used. No issues with SPS, LPS, Softies, or Coraline algae. They also dose trace elements...so you have to factor the costs of dosing that component seperately with the calcium to get a real cost comparative to other methods.
Scott