By Steve Bitter- The convict tang (Acanthurus triostegus
) is an attractive surgeonfish for many medium to large aquariums. It is silvery white with contrasting black or gray vertical bars, and adults can sometimes display a subtle yellow color particularly above the lateral line. This species stays smaller than many other surgeonfish species, but it is not one of the hardiest species. It is widely distributed throughout the tropical Pacific and Indian oceans. It is collected from a number of locales around the world, including Hawaii, Fiji and the Solomon Islands. There are, however, minimal color differences between specimens from differing areas.
Convict tangs are often delicate shippers and may be shy at first. This species is not a good tang for beginners, unless the specimen has already been acclimated to captivity, is swimming out in the open and is eating. Look for signs of external parasites and avoid specimens that are emaciated.
Adult convict tangs can commonly reach a size of 8 inches in captivity, and they should be kept in 125- to 180-gallon aquariums. They need to be fed frequently, which leads to substantial waste production, so be sure that there is plenty of filtration on your convict tang's tank. Good biological filtration is critical to keep ammonia and nitrite at undetectable levels (live rock is the best method in most cases). It is also important to have good water circulation to keep oxygen levels high and to allow outgassing of carbon dioxide. Running in-line mechanical filtration aids in filtering detritus, but filters must be cleaned regularly. Aim for a pH above 8.1 during the daytime. Nitrate should be kept below 20 parts per million through water changes or use of a refugium. Another especially helpful piece of equipment is an ultraviolet (UV) sterilizer. This will help clear aquarium water and can reduce the risk of disease.
The convict tang is known as one of the most peaceful tang species, which makes it ideal for fish-only displays with moderately high stocking densities. Good tankmates include wrasses, butterflyfishes, angelfishes, damselfishes, clownfishes and other community species. Predators, such as pufferfishes and lionfishes, are typically not a problem, as long as the convict tang is at least 50 percent the size of the predator. Keeping convicts with other tangs is unpredictable. Select species that are dissimilar in appearance; good choices might be the naso tang (Naso lituratus) or yellow-eye tang (Ctenochaetus strigosus). When mixing tang species, always watch for aggression and have a backup plan in case fighting occurs. This species is reef-safe, though its dietary requirements can make keeping water quality pristine a difficult proposition.
The biggest challenge in keeping surgeonfishes is duplicating their wild diet and feeding patterns. This species normally accepts normal aquarium fare, such as Mysis shrimp or flake foods, and these are good staples. In nature, however, convict tangs are roving grazers that move about the reef in huge schools rasping algae off of rock surfaces. This leads to constant snacking with very few big meals - the exact opposite of what most aquarium fish receive. To foster continuous eating, there are several options. Feeding stations can be useful, and these can be stocked with dried algae (nori) or freshly cultured macroalgae from a refugium. Many commercial grazing foods exist for this purpose, as well, and pellet or gel-based foods can help stretch mealtimes from seconds to several minutes. Fresh vegetables, such as romaine lettuce and zucchini, have been common food items in the hobby, but their nutritional value is much less than that of quality marine algae-based foods. Finally, automatic flake feeders or slow-release drip feeders can help make mealtimes more frequent and last longer. Whichever feeding strategies and food items you employ, make sure your tang is actively eating all of the food items you put into your tank.
Convict tangs have a complex social life cycle and travel in large schools as juveniles. One of the most impressive sights on a wild reef is a school of several hundred roving A. triostegus grazing together, leaving behind bare rock surfaces. It is a goal of mine to someday exhibit a large school like this in a public aquarium setting, but mixing multiple specimens together in smaller systems is often problematic, even for a short while. With this in mind, keep only one convict tang per tank, unless your system is several hundred gallons or more.
STEVE BITTER is a senior aquatic biologist at The Florida Aquarium in Tampa, Florida. His areas of interest include fish health and quarantine, scuba diving and coral reef restoration.