Sometimes aquariums smell. The buildup of skimmate within the protein skimmer collection cup can cause a rotten egg odor to emanate from your setup, old water smells can cause the carpet to stink, and that one missing fish that you couldn’t find is giving off that dead fish smell from behind the aquarium stand. Mold will even develop in a few places if the setup isn’t properly cleaned, which can also have a terrible smell. To get around these, cleaning the skimmer and providing proper air circulation in the aquarium room will certainly help. To help knock down on the smell of the protein skimmer, you can toss a bag of carbon onto the skimmer lid, and some models even have a carbon holding compartment. We would advise against using sprays or other odorizing chemicals to make the room smell nice, as these could have a significant negative impact on the aquarium inhabitants.