Convict Tang Care Guide: The Ultimate Reef-Safe Algae Eater

Species Overview#

The convict tang (Acanthurus triostegus) is the surgeonfish that breaks every rule the rest of its genus made. Where powder blues sulk, where achilles tangs pine, and where purple tangs declare war on their tankmates, the convict shows up calm, gregarious, and ready to graze. The five vertical black bars across a silvery white body look painted on, like prison stripes — hence the common name and the Hawaiian name "manini," which loosely translates to "small one of little consequence." Hawaiian reef fishermen used the species as a starter catch for kids learning to throw nets in the shallows.

In the trade, this is the rare tang you can actually keep in groups, the rare tang that won't redirect every grain of frustration onto a clownfish, and the rare tang that genuinely earns its keep as a working algae grazer. The catch — and there is always a catch with Acanthurus — is that convicts ship badly. They have the highest metabolic rate of any tang commonly imported, and a fish that arrives at your local fish store with a pinched belly is on a clock that runs in days, not weeks. Get the sourcing right and the rest of the care is forgiving.

Adult size

6-8 in (15-20 cm)

Lifespan

10+ years

Min tank

75 gallons (125+ for groups)

Temperament

Peaceful schooling

Difficulty

Intermediate

Diet

Herbivore

The "Manini": Origin and Natural Schooling Behavior#

Convict tangs have the widest natural range of any surgeonfish — Hawaii through the Indo-Pacific, the Eastern Pacific, the Red Sea, and the coast of Baja California. They aren't a deep-water fish. Spend ten minutes snorkeling in chest-deep water on almost any tropical reef and you will see them, almost always in groups of dozens to hundreds, sweeping across rubble flats and turf-algae plateaus.

The schooling behavior is not optional in the wild. Convicts use sheer numbers to overwhelm the territorial damselfish that defend prime algae patches — one or two convicts get chased off, but a wave of forty cannot be repelled. This translates directly to captivity. A solo convict tang in a community reef will be nervous and slow to color up. A group of three to seven in a long tank will spend their day cruising in formation, grazing in coordinated passes, and behaving exactly like the wild fish they came from.

For the broader pattern of which tangs cohabitate and which do not, see our yellow tang care guide and tomini tang care guide — both species can co-exist with convicts when introduced together in adequate footprint.

Identification: Vertical Stripes and Silver Body#

Identification is one place the convict gives you no trouble. Five solid black vertical bars run from the dorsal surface down past the lateral line on a silver-white background, with a sixth shorter bar on the caudal peduncle near the tail base. The body is more elongated and laterally compressed than the disc-shaped yellow tang or the diamond-shaped achilles tang.

The peduncle scalpel — the modified spine that gives surgeonfish their common name — is small but sharp on convicts and folds flat against the body when not deployed. Handle with a soft net and never bare hands; the scalpel can lay open a finger and is coated with a mild toxin that makes the cut sting for hours.

Juveniles look essentially identical to adults, just smaller and a bit more vivid. There is no dramatic adult color shift the way you see in regal angels or emperor angels.

Maximum Size and Lifespan (8 inches / 10+ years)#

Wild convict tangs reach about 10 inches; aquarium specimens typically top out at 6 to 8 inches. They grow steadily for the first two years, then slow dramatically. A well-fed convict in a properly sized tank will easily live a decade, and there are documented public-aquarium specimens past 20 years.

The two factors that cut lifespan short are inadequate diet (leading to chronic head and lateral line erosion) and insufficient swimming space (leading to stress-suppressed immunity and recurring ich outbreaks). Neither is hard to avoid if you commit to the basics.

Water Parameters & Tank Requirements#

Convicts are reef-water fish and they expect reef-water stability. They tolerate the standard mixed-reef parameter window without issue, but they punish swings hard — a 0.3 pH drop overnight or a temperature spike of 4°F can trigger an ich outbreak the next morning.

Minimum Tank Size (75-gallon minimum; 125+ for groups)#

A single convict tang needs a 75-gallon tank with a minimum 4-foot footprint. Volume matters less than length here. A 75-gallon long (48"x18"x20") works; a 75-gallon tall (24"x24"x30") does not, despite the identical water volume. These fish swim laps for ten hours a day, and a tank that forces them to turn every two seconds is a tank that produces a stressed, short-lived fish.

For a school of three or more, step up to a 125-gallon (72"x18"x22") at absolute minimum, and a 180-gallon (72"x24"x24") is far better. The extra width matters because it gives subordinate fish room to graze without crowding the dominant individual's lane. If you are deciding on tank footprint before stocking, our saltwater aquarium guide covers the relationship between tank length and stocking density for active swimmers in detail.

Flow and Oxygenation: Recreating High-Energy Reef Zones#

In the wild, convict tangs live in the surge zone — the shallow band of reef where wave action is constant and dissolved oxygen sits near saturation. Replicate this in captivity with strong, broken flow from two or more powerheads (Gyre-style flow makers work well) plus surface agitation from a return pump or wavemaker positioned to break the water surface.

Total tank turnover should be 30-50 times per hour for an active tang display. Dead spots in the rockwork are fine for the corals but should not dominate the upper water column where the tangs cruise. Surface skimming via an overflow weir helps oxygen exchange and removes the protein film that builds up in any heavily-fed tank.

Specifics: 72-78°F, pH 8.1-8.4, dKH 8-12#

The standard reef parameter window applies. Hold temperature at 76-78°F and resist the temptation to let it drift up to 82°F in summer; metabolic demand climbs with temperature and a hot tank means a hungrier, more stressed fish.

A note on nitrate and phosphate — many reefkeepers chase ultra-low-nutrient systems, but a convict tang in a starved tank is a hungry tang. Some background nutrient lets film algae regrow on rocks faster than the tang can clear it, which is exactly what you want for a grazer's daily diet.

Diet & Feeding#

This is the section that determines whether your convict tang lives or dies. There is no other way to put it. Tangs starve faster than almost any other reef fish, and the convict starves faster than any other tang.

The Importance of Marine Macroalgae and Nori#

The base of the diet has to be marine macroalgae — never freshwater spinach, never lettuce, and never freshwater "algae wafers" sold for plecos. Sheets of dried nori (the same product used for sushi, but unseasoned and unsalted) clipped to the tank glass with a magnetic veggie clip are the most reliable daily staple. A 6-8 inch convict will work through a half-sheet of nori per day, often in a single afternoon grazing session.

Rotate species to broaden the nutritional profile: green nori, red nori, purple nori, and chunks of fresh chaetomorpha or ulva harvested from a refugium. Variety matters because each macroalgae carries a different micronutrient and pigment load, and a monodiet of green nori produces a fish with washed-out coloration over six to twelve months.

Supplementing with Vitamin-Soaked Pellets (Selcon)#

Macroalgae alone is not nutritionally complete. Supplement with a high-quality herbivore pellet (New Life Spectrum Algaemax, Hikari Marine A, or Two Little Fishies SeaVeggies pellets) once daily, and soak the pellets in Selcon (or another HUFA/vitamin supplement) two or three times per week. The fatty acids and stabilized vitamin C in these supplements directly prevent head and lateral line erosion, which is the most common chronic disease in long-term tang husbandry.

Frozen mysis or brine once or twice a week is fine and the convict will eat it eagerly, but it should be a treat — not a staple. A diet weighted toward animal protein gives a tang the wrong gut flora and can produce digestive issues that look like a wasting disease.

Grazing Frequency: Why They Need to Eat All Day#

Convict tangs evolved to graze almost continuously during daylight hours, taking small bites of low-calorie algae for ten or twelve hours straight. Their gut is short and processes food fast, which means a single large feeding per day is not enough — the fish hits the bottom of an empty stomach within an hour and starts catabolizing muscle.

The practical solution is the nori clip plus regular small feedings of pellets and frozen food morning and evening. If you run a refugium with chaetomorpha or gracilaria, the tang will visit the overflow and pick at strands escaping into the display, which closes the gap nicely.

Tank Mates & Compatibility#

Compatibility is where convicts shine relative to other tangs. They are the easiest Acanthurus to integrate into an established community, and they can usually be added to a tank that already houses a different tang species without the all-out warfare you would see introducing a yellow or purple tang into the same situation.

Reef-Safe Status: Corals vs. Invertebrates#

Convict tangs are 100% reef safe with corals — SPS, LPS, soft corals, zoanthids, and clams are all ignored. They will graze film algae directly off the bases of frags and even off coral skeletons without harming polyps. This is the single most underrated feature of the species.

Inverts are also safe. Convicts ignore cleaner shrimp, peppermint shrimp, hermits, snails, urchins, and starfish. The only edge case is very small, freshly-molted shrimp that wander into a hungry tang's path, but this is rare and not species-specific.

For pairing convicts with cleanup-crew workhorses, see our cleaner shrimp care guide and turbo snail care guide — both make excellent companions and complement the tang's grazing duties.

Conspecific Aggression: Keeping Convicts in Groups#

Unlike most surgeonfish, convicts can and should be kept in groups. The trick is the introduction protocol. Add all individuals simultaneously, ideally as juveniles of similar size, and add them last in the stocking sequence so no single fish has had time to claim the tank as territory. Introduce two convicts and the dominant one will harass the subordinate to death; introduce five and aggression diffuses across the group.

The minimum group size for stable in-tank schooling is three, and five to seven is genuinely better. A 180-gallon tank can comfortably support a school of seven adult convicts plus a mixed reef community without any of the inter-tang aggression you would expect from other Acanthurus species.

Choosing Peaceful vs. Aggressive Tank Mates (Avoid similar body shapes)#

For the rest of the community, convicts pair well with ocellaris clownfish, royal gramma, yellow watchman goby, banggai cardinalfish, and most peaceful wrasses including the six-line wrasse. They generally ignore coral beauty angelfish and other dwarf angels, and coexist well with bicolor blenny and other algae-grazing blennies (though there will be some posturing over premium algae spots early on).

Avoid housing convicts with aggressive triggerfish, large eels, or hostile damselfish — none of these will kill a healthy convict outright, but the chronic stress will suppress the tang's immune system and invite disease.

Common Health Issues#

Tangs as a group are notorious for two diseases — marine ich and head and lateral line erosion. The convict gets both, and the convict gets the first one harder than most.

Marine Ich (Cryptocaryon irritans) and Velvet Sensitivity#

Cryptocaryon irritans — saltwater ich — finds tangs the way a magnet finds iron filings. Convict tangs have thinner mucus coats than many reef fish and a shipping-stress profile that lands them in your tank already immunocompromised. A tank that has been ich-free for years will sometimes break out two weeks after a new convict is added, because the convict was carrying a sub-clinical infection that flared once it settled into the new environment.

The non-negotiable answer is a 30-day quarantine in a bare-bottom QT tank with copper therapy at therapeutic dose (around 2.0 ppm with a Hanna copper checker, dosed slowly over the first 48 hours to avoid copper shock). Tank-transfer method is an alternative for hobbyists uncomfortable with copper, but it requires more discipline and more equipment.

Marine velvet (Amyloodinium ocellatum) is a separate parasite and is far more lethal than ich — a convict that contracts velvet untreated will be dead inside 96 hours. Velvet presents as a golden-dust film over the body and rapid breathing. Treatment is the same as ich (copper or chloroquine phosphate in QT) but the timeline is unforgiving.

Lateral Line Erosion (HLLE) and Nutritional Deficiencies#

Head and lateral line erosion (HLLE) is the most common chronic disease in tangs that survive their first year. It presents as pitting and erosion of the soft tissue around the eyes, on the forehead, and along the lateral line down the body. Once advanced, the damage is partially reversible but the fish will rarely look truly healthy again.

The cause is multifactorial — vitamin deficiencies (especially vitamin C and HUFAs), poor water quality, stray voltage from old powerheads or heaters with cracked seals, and chronic stress all contribute. Prevention is far easier than cure: vary the diet aggressively, soak food in vitamin supplements two to three times weekly, run carbon to remove tannins and dissolved organics, and replace any aging powerheads showing signs of voltage leakage. A grounding probe is cheap insurance.

The Importance of a 30-Day Quarantine Protocol#

A proper QT setup is a 20- or 29-gallon bare-bottom tank with a sponge filter, heater, PVC fittings for hiding, and dim lighting. Run prophylactic copper for two weeks (after a 48-hour observation period to confirm the fish is eating), then a two-week observation period with no medication to confirm no parasites rebound. Total elapsed time is 30-35 days.

This is also the right time to fatten the fish back up. Heavy feedings of nori, pellets, and Selcon-soaked frozen food in QT will rebuild the body condition that shipping stripped away, and a fish entering your display tank with a full belly is dramatically more likely to thrive long-term.

Where to Buy & What to Look For#

The convict tang you bring home will determine whether the next ten years are joyful or tragic. Spend the time on this step.

Identifying "Pinched Stomachs" in Local Fish Stores (LFS)#

The LFS health check for a convict tang has three checkpoints. First, look at the belly profile from the side — a healthy convict has a gently rounded ventral curve from the gill plate to the vent. A "pinched" convict shows a concave hollow behind the head and a sharp angle into the body, both signs of advanced wasting that often cannot be reversed.

Second, watch how the fish swims. A healthy convict moves continuously and smoothly, rarely stopping for more than a few seconds. A sick or dying convict hovers in one spot, lists slightly to one side, or breathes through one gill (a nearly-certain indicator of velvet or gill flukes). Ask the LFS staff to feed in front of you — a healthy convict attacks food immediately and aggressively.

Third, count the stripes and look for clean edges. The five vertical bars should be sharp-edged and uniformly black, not faded or smudged. Any pale patches between the stripes, golden-dust film over the body, or visible white spots are immediate disqualifiers no matter how good the price looks.

Sourcing Sustainable, Cyanide-Free Specimens#

The convict tang is widely collected throughout its range, and collection methods vary dramatically by source country. Hawaii-collected manini are typically net-caught by experienced divers under state regulation and arrive in excellent condition. Indonesia and the Philippines have a mixed record — most reputable wholesalers have moved to net-caught sourcing, but cyanide-collected specimens still appear and are nearly impossible to identify until they die unexpectedly two months in.

Ask the LFS where their tangs come from, and prefer dealers who can answer that question with confidence. The premium for known-source specimens is usually $20-40 over the cheapest option and is the best money you will spend on this fish.

Quick-Reference Cheat Sheet#

The convict tang rewards good husbandry with a decade or more of active, schooling, algae-grazing display behavior in a way that almost no other surgeonfish can match. Get the sourcing right at the LFS, run a real quarantine, give them a long tank with strong flow and constant grazing food, and the species essentially takes care of itself. Skip any of those steps and you will join the long line of reefkeepers who quietly conclude that "tangs are hard." They are not. They are specific.