Some sharks can breathe while remaining motionless on the seafloor. Others must swim in order to pump the water over their gills. For obvious reasons, sharks that can pump water over their gills while remaining motionless are better suited for the home aquarium. They don’t have to swim constantly and a lot of these types of sharks are relatively small (see horn shark, Port Jackson shark, etc). Don’t let their size and inactivity fool you though. These sharks still need a lot of room to move around and be healthy.