Carbon cleans up your water by absorbing the fish waste. It holds on to it until full then releases it back into the water ten fold. By the time you realize this is happening it could be too late. You're gonna have a spike and the fish wont be able to handle it. Also, if you plan on using carbon you'll rob your biological filter of the stuff that keeps it running strong-fish waste. Just remember, carbon has a very short life time. A fully matured biosystem can outlast the life time of most of your fish. Good luck with your Discus Monsta.