While there are a number of soapfishes that enter the aquarium trade, aquarists most commonly encounter the sixline soapfish (Grammistes sexlineatus). This boldly marked, handsome fish attains a maximum length of around 12 inches, and is found from the Red Sea to Marquesas and Mangareva, north to southern Japan and south to New Caledonia. The coloration of this species changes throughout its life. Individuals of around 0.5 inches or less have a coal black body with whitish-yellow spots, and they look similar to a noxious sea slug in the genus Phyllidia. This color pattern, which possibly serves to warn predators of the soapfish’s distasteful body slime, even stands out in the low light conditions that occur under ledges and in. In individuals between 2.5 to 3.5 inches, there are three yellowish-white longitudinal stripes, while those over 3.5 inches have six white stripes. In large adults (larger than approximately 6 inches) the stripes are even more numerous but are broken up into dashes.
The sixline soapfish is found on lagoon patch reefs, reef flats, fore reef slopes, near out-croppings on muddy slopes, and on occasion, in estuaries. It has been reported at a depth range from less than 3 to 400 feet. This species exhibits some degree of size-related depth segregation, with juveniles occurring more frequently in shallow water and large adults being limited to greater depths. The sixline soapfish is a solitary species that lives in holes, crevices and under ledges. It usually stays within, or very close to, a hiding place during the day; but at dusk it will travel further afield to hunt. It feeds heavily on crustaceans, such as crabs, juvenile crayfish and shrimp, but also eats small fish. When the sixline soapfish approaches a potential prey item, it will roll on its side, presenting its quarry with a view of its back only. This gives the impression that the soapfish is not as large as it would appear from the side. The soapfish approaches its prey slowly and may rapidly shake its head from side to side right before the final strike. Aquarists have reported having fish approach a newly introduced sixline soapfish to be “cleaned,” possibly mistaking the white longitudinal stripes of this fish for that of a cleaner species.
Smaller sixline soapfish may mimic several of the more common cardinalfish species (e.g., Apogon novemfasciatus and A. nigrofasciatus). The soapfish is not only similar in appearance to these fishes, but it is also found in the same habitat and may even associate with them. If a mimetic relationship does exist between these species, it is probably a form of aggressive mimicry, where the soapfish is able to approach prey items that are not threatened by the less voracious cardinalfishes.
The sixline soapfish is a durable aquarium fish with an incredible appetite. It will eat any tankmate (fish or crustacean) that will fit into its capacious mouth, including prey items almost as large as it is. It has even been known to eat other groupers and members of its own species. The sixline soapfish will eagerly suck up live feeder fish and live ghost shrimp, and can usually be coaxed into accepting chunks of fresh seafood, krill or cubes of frozen preparations. Adults can be housed in tanks as small as 55 gallons, and they can be kept at water temperatures of 72 to 80 degrees Fahrenheit.