Todd,
The initial work I've done indicates that it is carbon dust that can cause HLLE in some fish, not that it removes anything vital from the water for fish. In regards to carbon removing vital elements for corals, I don't have a firm opinion on that, but agree that it is probable.
My advice is to use a good grade of carbon (pelletized perhaps) rinse it very well before use, and only use it in tanks with protein skimmers (because skimmers are very effective at removing POC, including carbon dust).
Jay