Photo: Felicia McCaulley Dwarf seahorses, like this cute example, are named for their tiny size: they only reach about two inches long when fully grown. They are the slowest swimming fish, taking an hour to move just five feet! Dwarf seahorses are a popular breed to keep in fish tanks, where they can live for up to two years. And if you're fond of baby seahorses, they're the species for you! Unlike some other marine wildlife, dwarf seahorses will also happily breed in captivity.