The spotted sea hare, Aplysia dactylomela, is one of the larger and more animated of the sea slugs.
In the wild they can get as long as maybe a foot, but in tanks they usually won’t get very big. The ones we sell are about 2″” to 4″” long and are a nice size for most tanks. They only eat algae (and lettuce), so you don’t have to worry about them eating corals or other invertebrates. Not much seems to bother them, but if they are disturbed they can release purple ink. I don’t believe the ink is toxic, but it will be removed by carbon filtration, and it doesn’t seem to cause any problems to their tank mates.