A carbon filter will eliminate pretty much all smell as long as the unit is air tight (apart from intake) and that there is a good air flow out the other end via the filter. You may find that if u run the filter on the same switch as ur lights the smell will leak out during the dark period but shouldnt be anywhere near the level that you will get from not having a filter at all. Usually the problem is made worse because the room obviosuly has to be vented so you are in effect creating a huge air freshening chamber churning out that lovely pot smell lol I've had to invest in a carbon filter recently cause the house was a joke, everyroom stank, it has been amazing, best piece of kit i've bought in ages!