I have seen cases that demonstrate carbon as a cause of HLLE
in freshwater fish, but this is rare. In marine fish it is well demonstrated to be a dietary issue.
Carbon can remove trace elements and micronutrients, which are both vital to fish and corals. Many people report their corals and plants (in planted tanks) doing better without carbon filtration. However, this is only if the tank is running so well that those trace elements and micronutrients are the limiting factor in the tank. There is always a limiting factor, but in most tanks it is not these things. In the tanks where these are not the limiting factor I think carbon is worth running, but still not essential. Most of the issues that carbon takes care of can be better handled with water changes.