I pretty sure activated carbon is fine as long as you change it regularly, there is always the problem of leching contaminates into the tank with old carbon that's why you need to change it. I change my carbon evry three weeks which is half the time of 4-6 weeks recomended by the mfg. So from my understanding carbon can be a bad thing on any tank if it is not changed often but I would think as long as you are changing it on a regular basis like I do once evry three weeks you should be good. I am not an expert on Discus but I never heard not to use it before now that is. If you do not use AC then what keeps the tank from smelling or getting that yellow tinge that the AC is suppose to get rid of in other words what is the substition and if there is none dose anyone run into rpoblems of yellow tank and smelly for those of you who do not use the AC is there any draw backs from not using it?