I've heard so many bad reports about charcoal that I don't use it at all anymore. Some people say that once it has reached its capacity, it will release everything back into the water (unsure about this one), but the micro-cavities definitely do fill up quite quickly. My advice to anyone using charcoal is to only use it as you need it (e.g. to remove medicines after treatment) and then bin it after <1 week.
Personally, my first treatment for fish is always salt and then maybe Melafix. Since both are 'organic' treatments which are relatively mild compared to other treatments, I don't need to rush to remove it from the water. But even then, I prefer to remove them through regular water changes instead of using charcoal. Some people also say that prolonged exposure to charcoal leads to hole in the head disease in cichlids, that's why most cichlid-keepers tend to avoid it, just in case.
If you're thinking about using charcoal as a biological media, ceramic noodles are a much safer and cheaper option (in the long run).