Gem tang
Zebrasoma gemmatum
One of the most prized and rare surgeonfishes in the marine aquarium hobby, featuring a spectacular black body adorned with white spots and a yellow tail.
Water Temperature
22 - 28 °C
pH Value
8.1 - 8.4
Water type
Marine
Tank level
All levels
Adult size
22 cm
Minimum tank volume
>= 680 L
Minimum group
>= 1
KH
8 - 12 dKH
Species description
Geographical Origin and Habitat: This species has a restricted geographical distribution in the western Indian Ocean. It is primarily found around Madagascar, Mauritius, Réunion, and occasionally along the coastlines of Mozambique and South Africa. It inhabits deeper seaward reefs and drop-offs, typically at depths between 10 and 60 meters, where it grazes on benthic algae growth.
Taxonomy and Morphology: Belonging to the family Acanthuridae and the genus Zebrasoma, this fish displays a high, laterally compressed, disc-like body shape and a prominent snout suited for grazing. Like all surgeonfish, it possesses sharp, retractable spines (scalpels) on both sides of the caudal peduncle for defense. Adult specimens can reach a maximum length of approximately 20 to 22 cm.
Social Behavior: It is highly territorial and solitary, exhibiting severe aggression towards conspecifics and other member species of the Zebrasoma genus with similar body profiles (such as Z. flavescens or Z. scopas). It actively defends its feeding territory. However, it is generally peaceful towards other unrelated reef fish, making it an excellent candidate for large community reef tanks.
Coloration and Sexual Dimorphism: The coloration is striking and highly distinct: a deep black or dark brown body completely covered in tiny, gem-like white or light-blue spots. The caudal fin is a contrasting, vibrant bright yellow, and the pectoral fins show yellow accents. There is no visible sexual dimorphism; males and females appear identical, though females may grow slightly larger.
Care, breeding and tankmates
Aquarium Setup: As an active swimmer that establishes territories, it requires a minimum tank size of 400 to 500 liters (approx. 100-130 gallons). The aquascape should feature substantial live rock setups providing caves, swim-throughs, and visual barriers, combined with extensive open areas for swimming. Strong water movement is crucial to simulate their native high-energy reef environments.
Diet and Feeding: Herbivorous. While they will consume meaty foods like mysis and brine shrimp, their primary diet must consist of marine algae and seaweed. Provide dried nori sheets daily using an algae clip, supplemented with high-quality spirulina pellets, flakes, and vitamin-enriched herbivore preparations. A high-fiber diet is critical for their digestive health and vibrant coloration.
Water Quality: Demands pristine water conditions and high levels of oxygenation. Standard parameters should be: temperature 23-26°C (73-79°F), pH 8.1-8.4, specific gravity 1.021-1.025, and KH 8-12. Use an oversized protein skimmer and run active chemical filtration. Utilizing a UV sterilizer is highly recommended to manage waterborne pathogens and keep the fish healthy.
Compatibility and Cohabitation: Do not house with other Zebrasoma species in the same tank, as fatal fighting is likely to occur. It can coexist with surgeonfish of different genera (such as Acanthurus, Ctenochaetus, or Naso), anthias, angelfish, wrasses, and clownfish. It is completely reef-safe and will not harm corals or ornamental invertebrates.
Aquarium Breeding: Breeding in home aquaria is exceptionally difficult and has not been successfully achieved by hobbyists due to their pelagic spawning nature. Spawning events occur in large groups in open waters where eggs drift with currents. Some captive-bred specimens are beginning to emerge from professional aquaculture facilities, but they remain rare.
Risks and Diseases: Very susceptible to skin parasites such as Marine Ich (Cryptocaryon irritans) and Marine Velvet (Oodinium). A strict quarantine period is highly recommended. They can also suffer from Head and Lateral Line Erosion (HLLE) if subjected to poor diet, chronic stress, or high levels of dust from low-quality activated carbon.
Geographic Origin and Distribution
This species has a restricted geographical distribution in the western Indian Ocean. It is primarily found around Madagascar, Mauritius, Réunion, and occasionally along the coastlines of Mozambique and South Africa. It inhabits deeper seaward reefs and drop-offs, typically at depths between 10 and 60 meters, where it grazes on benthic algae growth.
Origin
Tropical oceans and reefsAfrica and MadagascarSouth and Southeast Asia
Fish profile
- Temperament
- Territorial
- The Gem Tang is considered semi-aggressive, particularly within the confines of an aquarium. While it generally ignores invertebrates and unrelated fish, it fiercely defends its grazing territory from perceived competitors. Its sharp caudal spines (scalpels) are effective weapons used during disputes. Providing a sufficiently large tank minimizes aggressive tendencies and promotes a harmonious community.
- Diet
- Herbivore
- The Gem Tang is primarily herbivorous, needing a substantial amount of marine algae in its daily diet. It should be offered dried seaweed (nori) attached to a grazing clip to mimic natural feeding behaviors. While it enthusiastically accepts meaty foods like mysis or brine shrimp, lacking sufficient greens can lead to Head and Lateral Line Erosion (HLLE). Spirulina-enriched flakes or pellets can also supplement its nutritional requirements.
- Life expectancy
- 10-15 years
- In captivity, this rare surgeonfish can live between 10 and 15 years when housed in a suitable environment and meticulously cared for. Its longevity is highly dependent on water quality and a rich, varied vegetable diet. Being an extraordinarily valuable species, it is essential to minimize stress and constantly monitor water parameters to prevent diseases.
- Adult size
- 22 cm
- The catalog's scalar value is 22 cm. The FishBase SPECIES table (v25.04 snapshot) source reports a maximum adult measurement of 22 cm for this species, recorded as total length (TL). The taxonomic record consulted is Zebrasoma gemmatum (v25.04); male or unsexed record. These figures are not interchangeable: the first is the profile's editorial reference, whereas the second is a published maximum under a specific measurement convention; differences among sources can reflect sex, population, life stage and measuring method.
- Sex ratio
- Solitary
- There is no easily distinguishable sexual dimorphism in the Gem Tang. Both males and females share the same striking coloration and identical morphological features. In a home aquarium, maintaining a specific sex ratio is generally irrelevant since keeping multiple individuals is highly discouraged. For breeding facilities, genetic testing or behavioral observation is required to confirm the sexes.
- Feeding frequency
- Twice daily
- Because of its highly active nature, this species requires multiple feedings throughout the day. Offering small meals 2 to 3 times daily ensures its high metabolic demands are met. A constant supply of grazing material, such as live macroalgae or dried nori, should be available between structured meals. Underfeeding can rapidly lead to weight loss and a weakened immune system.
- Bioload
- The Gem Tang (Zebrasoma gemmatum) has a moderately high bioload typical of active surgeonfishes. It constantly grazes on algae and requires frequent feedings, which translates to a steady production of waste. A robust filtration system, including a high-capacity protein skimmer, is necessary to manage its biological impact. Regular water changes are also critical to ensure nutrient levels remain within safe parameters.
- Flow
- Strong
- Coming from turbulent reef environments, the Gem Tang thrives in high flow aquarium setups. Strong water movement provides essential oxygenation and replicates the natural currents it encounters in the wild. Multiple powerheads or wavemakers should be positioned to create chaotic, multidirectional flow. This dynamic environment also encourages natural swimming and grazing behaviors across the rockwork.
- Jump risk
- No
- Reproduction
- Oviparous
- In the wild, Zebrasoma gemmatum are pelagic spawners, releasing eggs and sperm into the water column during synchronized events. They form large breeding aggregations, usually coordinating with lunar cycles and outgoing tides. Captive breeding of this tang is extremely rare and difficult due to the complex requirements of raising their microscopic larvae. Most individuals available in the hobby are wild-caught from specific locales.
- Compatibility & tankmates
- Semi-aggressive
- This tang is generally peaceful toward most non-conspecific tank mates in a reef setting. However, it can become highly territorial and aggressive toward other surgeonfishes, especially those of the same genus or similar body shape. It is best kept as the solitary tang or introduced simultaneously with other tangs into a very large aquarium. Careful observation is required during the initial introduction to mitigate severe aggression.
- Tank level
- All levels
- This species is an active swimmer that utilizes all levels of the water column. It will constantly patrol the open water seeking food and establishing its territory. When resting or feeling threatened, it retreats to the middle and lower levels, seeking refuge within the rockwork. Ample swimming space combined with accessible hiding spots is essential for its well-being.
Scientific Parameters & Models
FishBase
Window to the official scientific data of the species
Data synced on: 06/15/2026
- Trophic level
- 2 ± 0 se
- Resilience
- High (less than 15 months)
- Phylogenetic uniqueness (PD50)
- 0.5078
- Bayesian Length-Weight
- a = 0.02291, b = 2.96
- VulnerabilityLow vulnerability (12 / 100)
Image gallery
Licensed images linked to the species or, when marked, to the closest representative taxon.
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