The exposure time require for activated carbon to effectively remove chloramine is significantly higher than chlorine because the chlorine/ammonia bond needs to be broken for adsorption. Standard activated carbon is not very efficient at breaking this bond, there is however high grade catalytic carbon (as used in many household water filters) which does this reasonably effectively. It still needs a greater exposure time than for chlorine however and the other downside is once the bond is broken and the chlorine adsorped you're left with free ammonia which is not readily adsorped via carbon. Then you need to factor in the volume of the tank and the filter turnover and figure out what sort of chloramine reduction you're getting as the water passes through.