Black Skirt Tetra (Black Widow Tetra)
Gymnocorymbus ternetzi
The Black Skirt Tetra features a translucent body with a distinctive black triangular dorsal fin and prominent black markings on its flanks.
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About the Black Skirt Tetra (Black Widow Tetra)
Black skirts are those little "suit-and-tie" tetras with the dark bands and flowing fins that look way fancier than they should for how tough they are. They're super active midwater fish, and when you keep a proper group they do that tight, zippy schooling thing that makes the whole tank feel alive. Just give them enough buddies and finny tankmates they won't be tempted to nip.
Quick Facts
Size
3 inches
Temperament
Peaceful
Difficulty
Beginner
Min Tank Size
20 gallons
Lifespan
3-5 years
Origin
South America
Diet
Omnivore - good flakes/micro pellets plus frozen/live foods like brine shrimp, daphnia, and bloodworms
Water Parameters
20-26°C
6-8
5-19 dGH
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Calculate heater sizeCare Notes
- Don't buy just one-black skirts are schooling fish and get way less nippy/stressed in a group of 6+ (8+ is even better).
- Give them some open swimming room plus plants/wood around the edges; they love weaving in and out of cover but still want space to cruise.
- They're pretty forgiving, but keep the water steady: aim around 20-26°C (68-79°F), pH ~6.0-8.0 (many keepers target ~6.0-7.5), and don't let nitrates climb-regular water changes help keep them healthy and reduce stress.
- Feed small amounts 1-2 times a day; they do great on a quality flake/micro-pellet, and perk up fast with frozen foods like bloodworms, brine shrimp, or daphnia a couple times a week.
- Skip slow, flowy-finned tankmates (bettas, fancy guppies, long-fin angels)-black skirts can turn into fin-nipping little hooligans, especially in small groups.
- Best buddies are other quick community fish: danios, rasboras, corydoras, and most tetras that aren't tiny enough to be bullied.
- Watch for the classic "tetra fade" (pale colors, clamped fins) when the tank is cramped or the school is too small; bump the group size and add cover and it usually settles down.
Compatibility
Good Tankmates
- Other mid-sized, quick community fish (think larger tetras like rummynose or pristella, or a calm harlequin rasbora-type vibe). Black skirts are kinda bossy, so tank mates that can scoot around and don't get intimidated do best.
- Corydoras catfish (most types). They stick to the bottom, mind their own business, and black skirts usually don't bother them-just give the cories a group and some sand/soft substrate if you can.
- Bristlenose pleco (Ancistrus) or similar chill smaller plecos. Tough, not flashy, and they don't trigger the fin-nipping thing.
- Kuhli loaches or other peaceful bottom noodles. They're active at dusk, stay low, and aren't fin targets.
- Peaceful dwarf cichlids (e.g., rams, Apistogramma) in a spacious, well-structured tank; monitor for stress/fin-nipping and territorial behavior, especially during breeding.
- A proper group of black skirt tetras themselves (6+). They're way less annoying when they've got their own crew to bicker with-small groups are where the fin-nipping starts.
Avoid
- Slow fish with fancy fins (bettas, fancy guppies, long-fin mollies). Black skirts are classic fin-nippers and those flowing tails are basically a chew toy.
- Angelfish. Same issue: tall, slow-ish, long fins-black skirts will often stress them out and shred fins, especially in smaller tanks.
- Super shy, delicate fish (like small ember/neon-sized fish in cramped setups). Black skirts can be a little too much energy and can harass them when crowded.
- Other nippy/rowdy semi-aggressive fish that escalate things (tiger barbs, some serpae-type tetras). You can end up with a constant fin-war unless the tank is big and stocked really carefully.
1) Where they come from
Black skirt tetras (aka black widows) come from South America—slowish rivers and floodplain areas in places like Paraguay and southern Brazil. Think tannin-stained water, plants along the edges, and lots of little fish living in loose groups.
That background explains a lot: they like some cover, they look great over darker substrate, and they’re way more relaxed when you keep a proper group.
2) Setting up their tank
If you’ve only got room for one “easy tetra” tank, this species is a solid pick. They aren’t fussy, but they do have opinions—mainly about space and social life.
- Tank size: 20 gallons long is my happy minimum for a group. They’re active swimmers, not little hovercrafts.
- Group size: 6 is the bare minimum; 8–12 is where they really settle down and stop acting like jerks.
- Filtration: any decent filter works, but don’t blast them with a jet stream. Moderate flow is plenty.
- Temp: mid-70s°F (around 24°C) is a sweet spot. They handle a bit cooler/warmer, but stability beats chasing numbers.
- Decor: live plants (or fake if you must), some open swimming room, and a bit of wood/leaf litter if you like the blackwater look.
- Lighting: they don’t need it bright. Slightly dim + plants makes them bolder.
If your black skirts are pale and hiding, try this combo: darker substrate + floating plants + a bigger group. It changes their whole vibe.
Water parameters don’t need to be perfect-lab-grade. I’ve kept them in neutral community water without drama. The real trick is clean water and not letting nitrates climb forever—regular water changes keep their fins looking nice.
3) What to feed them
They’ll eat basically anything you put in the tank, which is great… and also how people end up with chunky tetras. Go for variety and don’t overdo it.
- Staple: a good-quality flake or small pellet
- Frozen: bloodworms, brine shrimp, daphnia (they go nuts for daphnia)
- Live (if you’re into it): baby brine shrimp, grindal worms, mosquito larvae where safe/legal
- Plant/roughage help: spirulina flakes or a veggie-based flake a couple times a week
Feed small portions. If food is still drifting around after a minute or two, you’re giving too much. Black skirts will absolutely convince you they’re starving—don’t fall for it.
4) Behavior and tankmates
Black skirts are classic semi-nippy tetras. Most of the time they’re peaceful, but if the group is too small (or the tank is cramped), they start “testing” fins. It’s not personal—they’re just schooling fish doing pecking-order stuff.
- Best tankmates: other sturdy community fish—corydoras, kuhli loaches, rasboras, danios, larger peaceful tetras, many dwarf cichlids, plecos (the smaller chill kinds)
- Use caution: slow, long-finned fish like bettas, fancy guppies, and some gouramis (nipping temptation is real)
- Skip: super timid nano fish that get stressed by their busy swimming
If you see fin-nipping, your first move isn’t “rehome the bully.” Try adding more black skirts (bigger school) and make sure you’ve got enough swim space and some plant cover to break up lines of sight.
One more note: they’re surprisingly zippy at feeding time. If you keep them with shy eaters, spread food around the tank or drop in sinking food for the bottom crew first.
5) Breeding tips (if you want to try it)
They’re egg scatterers, and they’ll happily eat their own eggs like it’s a snack platter. Breeding is doable, just not something that happens “accidentally” in a community tank unless you’re lucky.
- Set up a small breeding tank (10–15 gal) with a sponge filter and gentle aeration
- Use a spawning mop or clumps of fine-leaved plants (java moss works great)
- Condition adults with frozen/live foods for a week or two
- Add a pair or a small group in the evening; spawning often happens in the morning
- Pull the adults right after you see eggs (or use a mesh/marbles so eggs fall out of reach)
- Fry foods: infusoria/microworms at first, then baby brine shrimp as they grow
Males tend to be slimmer and may show a bit more fin extension. Females are usually rounder, especially when full of eggs.
6) Common problems to watch for
Most issues I’ve seen with black skirts come down to stress (too small a group, too little space, messy water) or classic community-fish diseases that show up when something slips.
- Fin nipping / torn fins: usually from small schools or cramped tanks; sometimes from bad tankmate choices
- Ich: often after a temperature swing or a new fish addition—quarantine new arrivals if you can
- Bloating/constipation: from overfeeding rich foods; give them a lighter day and add daphnia to the routine
- Faded color and hiding: commonly from bright lights, no cover, or being kept in too-small groups
- “Mysterious deaths” in new tanks: they don’t like unstable cycling—ammonia/nitrite will wipe them out fast
My quick health check: watch the group at feeding time. If one hangs back, clamps fins, or breathes fast, that’s your early warning to test water and look closely for spots or frayed fins.
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