It acts as a buffer and raises ph. I used it in my African cichlid tank. I never noticed any side effects due to it. It would also likely raise calcium levels.
From what I understand it will only act as a buffer (and increase PH) when the PH is at lower end of what we shoot for in aquarium... i think somewhere below 7.8? Based on that I dont think it would do much for most of our saltwater tanks other than in something like a calcium reactor where we are forcing PH down in order to "dissolve" it.
What mdb said. CC will keep saltwater pH from getting too low because it will dissolve when mild acids are present. However, it only buffers at the low end.
You can actually use CC in calcium reactor.
I've seen SPS tanks that don't dose or do water changes with deep CC beds. As gases and organics slowly dissolve the CC it acts like a slow burn calcium reactor. Once corals acclimate to the low alk and calcium levels all is good. Very stable tanks but not for everybody.