Aquatic Ambiance

Despite the fact that most fish do not have lungs, they are still able to “cough.” When a fish’s gills are being obstructed by parasites (such as water fleas and isopods), the fish rapidly flutters its gills in an attempt to bring more water across its gills (aiding in oxygen acquisition and potentially dislodging the obstructions). Fish also do this when they are plagued by a disease or ingest a pollutant that causes mucous build-up.

(Source(s(s))