Freshwater Fish · Tetras

Black Skirt Tetra

Gymnocorymbus ternetzi

Hardy deep-bodied shoaler with a dark finBeginner

Lifespan: 3-5 years
Size: 2-2.5 in
Social needs: Shoaler; keep 12+ together
Time: Low; daily feeding, weekly water change
Cost: Low
Diet: Omnivore; flakes, pellets, frozen foods

Overview

What is this animal?

The black skirt tetra is a deep-bodied characin from the Paraguay and Guapore river basins, recognised by its silvery-grey body, twin black bars and dark flowing anal fin.
Larger and sturdier than most tetras, it is a long-standing community favourite.

It is hardy and adaptable, tolerating a range of conditions, which makes it beginner-friendly.
Be wary of dyed or injected colour variants, which raise welfare concerns and often show reduced vitality.

Temperament

Generally peaceful and lively, black skirts can become nippy, especially toward long, flowing fins, when kept in groups that are too small.
Seriously Fish advises a shoal of at least twelve, which diffuses this behaviour and keeps them calmer.

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They suit lively community tanks with similarly sized, robust species and open swimmers.
Avoid pairing them with slow, long-finned fish such as bettas or fancy guppies, and keep twelve or more to reduce fin-nipping.

A good fit for

Beginners wanting a hardy, forgiving shoaler
Active mid-sized community aquariums
Keepers in areas with variable tap water
Those who like a bold, contrasting fish

Common mistakes to avoid

Fin-nipping when kept in too-small groups
Pairing with slow, long-finned tankmates
Buying dyed or injected colour variants
Undersized tanks limiting their active swimming

Enclosure

Minimum 20 gallons, for a group of 12

Cannot be kept alone. The minimum is 12, and the space above is for that group.

Keep a shoal of at least twelve in a tank at least 30 in long holding 20 US gallons or more, filtered and heated to 72-81°F.
Their adult size and active swimming call for more horizontal space than tiny tetras, with planted edges and driftwood framing open swimming room.

They accept a range of soft to moderately hard water but value stability most.
A well-cycled tank with steady parameters and gentle to moderate flow suits their robust temperament.

Check your enclosure

Supplies

Setup

Tank and stand
Filter
Heater and thermometer
Substrate, plants and decor
Liquid test kit
Dechlorinator and starter supplies

Ongoing

Micro-pellet or finely crushed tropical flake
Frozen daphnia and cyclops
Frozen or live baby brine shrimp
Microworm culture (optional, for fry and conditioning)
Spirulina-based flake

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Fine net for removing leftovers
Small feeding spoon for accurate pinches

Safe substrate: Fine, smooth sand · Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras) · Bare bottom (quarantine and grow-out only)

Work out the cost

Care routine

Daily

Check temperature and equipment
Feed
Head-count and look everyone over

Every few days

Top up evaporationevery 3 days

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Weekly

Count the school
Test the water
Water change and gravel vacuum
Wipe the front glass only

Every few weeks

Trim and thin the plantsevery 2 weeks

Monthly

Check supplies and reagent dates
Replace or recharge chemical media, if you use any (skip it in a blackwater tank)
Rinse the filter media

Health review: Quarterly school and stability review, every 3 months

Tools

Is your enclosure big enough?Check your measurements against the recommended size What will it cost?Setup, monthly and over its whole life, itemised Is this ammonia reading dangerous?Works out the toxic part from your pH and temperature

Care in depth

Diet

Omnivore; flakes, pellets, frozen foods

Black skirts are unfussy omnivores.
A quality flake or pellet forms a reliable staple, rotated with frozen or live bloodworm, daphnia and brine shrimp, plus occasional plant-based or algae food for balance.

Feed small amounts once or twice daily, only what is consumed in a minute or two.
They eat enthusiastically and can outcompete shy tankmates, so make sure slower feeders also reach the food.

Health

Signs of good health, and the common problems

These are among the more disease-resistant tetras, with most problems traceable to chilling, poor water quality or stress.
Ich and fin rot are the usual concerns and respond well to corrected conditions and early treatment.

Long-finned varieties are more prone to fin nipping and damage.
Quarantine new stock, maintain weekly partial water changes, and avoid dyed or injected colour-variant fish, which carry higher disease and mortality risk.

Is this urgent?

6 warning signs and how fast to act, 3 of them right away

In a fish tank the patient is usually the water. Test before you treat: dosing a healthy tank for a disease it does not have makes things worse.

Go now

Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.

Now

Fish gasping at the surface with fast gill movement, or crowding the filter outflowIncrease surface movement now (gently in a betta or gourami tank, without creating a strong current), then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems. Bettas, gouramis and corydoras rise to gulp air as part of normal breathing; the warning is constant gulping, fast gill movement, or the whole group hanging at the top.

Now

Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths usually point at the water or the equipment rather than a pathogen, so rule those out first.

Now

Today

Flashing: repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.

Today

White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.

Today

Soon

Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.

A few days

Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.

This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it, and the time to find one is before you need one.

Bedding and substrate

3 safe, 4 with a caveat, 2 to avoid

Substrate in an aquarium is a filter as much as a floor. Most of the biology lives in and on it.

Safe

Fine, smooth sandCorrect for corydoras, kuhli loaches, eartheaters such as rams and acaras, and anything else that sifts or burrows. Barbels wear away on sharp gravel.

safe

Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras)Rinse thoroughly before use. Grain size should be too large to swallow and too small to trap food. Kuhli loaches need sand to burrow into and rams and acaras sift mouthfuls of it; bronze and panda corydoras manage on smooth fine gravel, but sterbai corydoras need sand.

safe

Bare bottom (quarantine and grow-out only)Legitimate for quarantine and grow-out tanks. Easy to clean, but it holds far less biological filtration, and it is no long-term home for burrowing loaches or sand-sifters.

safe

With a caveat

Crushed coral or aragoniteDeliberately raises hardness and pH. Correct for rift-lake cichlids, wrong for soft-water species.

with a caveat

Aquasoil for planted tanksExcellent for plants, but most active soils soften the water and pull pH and KH down for months, and many leach ammonia for the first weeks. Right for soft-water species; wrong for hard-water animals (livebearers, rainbowfish, goldfish, and snails, whose shells erode in soft, acidic water) unless you test GH and KH and remineralise. Cycle around it; do not stock into it.

with a caveat

Very deep sand bedsAnaerobic pockets form below about 2 inches in still sand. Either keep it shallow or keep it stirred.

with a caveat

Painted or coated gravelUsually fine, but coatings chip. Test a handful in a jar for a week first.

with a caveat

Never

Sharp or angular gravel with bottom-dwellersWears down barbels and leads to infection in corydoras and loaches.

never

Anything containing metal, limestone of unknown origin, or shells in a soft-water tankSlowly alters chemistry in ways that are hard to diagnose later.

never

This is our general list for freshwater fish. We have not built one specific to Tetras yet.

Top resources for more info

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