A properly designed and maintained aquaponic system should not produce any foul smells.  When growing fish, you can expect a slight odor, but it should not be offensive.  If you have an effective fish waste solids management system (meaning an adequate bacteria population and/or mechanical solids filtration) you should not expect your system to be very smelly.  A healthy, aerobic (oxygenated) aquaponic system is what we strive for, and will not smell any more than the smell of a healthy soil garden.  Anaerobic zones (oxygen-lacking) within the growbeds, pipes, or other areas of your aquaponic system will cause a foul, sulfur smell.  It will be necessary to unclog any anaerobic zones and possibly add air stones to the area to keep the system healthy and odor-“less”.
Often a dead fish in your tank may be the culprit of a foul smell, so be sure to check in on your fish frequently and ensure that you have easy access to the fish tank.