The air fern is not a fern at all, as the name implies. In fact, it is not even a plant. Although it looks like a fern and may even be dyed green, an air fern is really the remains of a sea creature called sertularia. This creature is related to coral, and its skeleton looks much like a fern. Air ferns are dredged from the sea, treated with chemicals and marketed as "plants" that don't need air or water. It's true the dead remains of the sertularia certainly don't need to be watered. In fact, if you water it, you may wash off the dye and cause it to begin to smell. Instead, care for your air fern in other ways.
Air out the fern. Sea ferns are often enclosed in boxes for shipping. The chemicals used to treat the remains are quite odorous and the smell can become strong when the items are boxed up. Place the fern outside (where it will stay dry) and let the smell dissipate before displaying it.
Submerge your air fern in water immediately upon unpacking it if you are going to use it in an aquarium. In this case, you do not have to air out the air fern. Note that air ferns made for aquariums are treated differently than those that are not meant to get wet, so make sure your air fern is made for aquariums before you immerse it in water.
Keep your air fern away from direct sunlight, which can bleach the dye out of the fronds and increase the potency of the sometimes unpleasant smell.
Check the air fern often for cobwebs and dust. Do not vacuum it, as the air fern is too delicate, but you can lightly dust it with a feather duster.