This black cloud is just dust from the carbon. It is relitively harmless, so long as no fish are in the tank. The filters will soon clear it again, so it will only have minimal (if any) effect on tank occupants anyway. Next time you have to use carbon, it would be a good idea to rinse it under the tap before placing it in the filter. This will clear the dust, and prevent the cloud. I should also make you aware that the carbon must be removed/replenished after arround 4 weeks, or it will dump anything it has absorbed back into the tank. Carbon is not nessisary, and is only usualy used to remove tannings and medications from the water.
HTH
Rabbut
