Funny but I can't say my rats really smell much unless I'm lax on cleaning their bins weekly. Basically it's good ventilation (I like my side ventilated bins for that), no overcrowding, full change once weekly and if it is a pretty full bin I do a couple of big handfuls of clean bedding mid-week just to freshen it up a bit. If I give them some kitchen leftovers as a treat I just check the next day and remove anything uneaten so it doesn't go nasty if they hide it in the bedding.