There was an article some months back that linked activated carbon to the occurrence of head and lateral line erosion disease (HLLE) in tangs. The study found that even tangs that showed no visible symptoms had microscopic deterioration of the tissue along their lateral lines when kept in aquariums with carbon, while tangs that were kept without carbon had no signs, even at the microscopic level.
Mind you, this was only one study, but it made quite a splash when it came out. The authors suggested that the primary culprit was microscopic carbon dust that was dislodged from the media and spread throughout the water, so they suggested buying the highest grade carbon that produces the least amount of dust possible, has been thoroughly rinsed, and then running it in a filter or reactor that doesn't agitate it at all.
Also, some brands of carbon are made from plant material that can be loaded with phosphates which leach in to the water, while other brands are supposedly 'phosphate free'. I've personally experienced the effects of cheap carbon in a freshwater aquarium:
This photo was taken of a stable, 2 year old planted fresh water tank. I took it on a Friday right before i did a small water change and replaced the carbon in the filter with a much cheaper brand. The tank was in my old office and it was a long weekend.
This is what it looked like when I came back in on the Tuesday. It took weeks to get it back to normal. Thankfully the Discus survived.
I would recommend buying the best carbon you can if you're going to run it.