Coal has all types of other chemicals mixed in with the carbon; also, the carbon is arranged in a tight bonding system that will not absorb anything but it will put out some very toxic substances which could kill your fish.

Is charcoal harmful to fish?

Charcoal filters are generally regarded as safe, but some controversy surrounds their long-term use. Charcoal filters are great at sucking bad stuff out of the water, but they can also absorb trace minerals that are important for healthy fish and plants. ... Discus fish and oscars are particularly susceptible.

Is charcoal good for fish?

Charcoal filters out undesirable chemicals and elements in the water of your aquarium. These substances include dissolved organic molecules found in tap water, chlorine and chloramine, some heavy metals, and growth-inhibiting pheromones released by your fish.