The black skirt tetra, also sold as the black widow tetra, is a deep-bodied South American tetra that tops out at around two and a half inches. Young fish wear bold black bands over silver, and the pattern softens towards grey as they mature. They are mid-water schoolers with a strongly social streak: kept in a proper group they mostly bicker among themselves, but a lonely black skirt or a tiny trio often redirects that restless energy onto slower tank mates, which is where most fin-nipping stories about this fish begin.
Why black skirts get their nippy reputation
The nipping is social behaviour, not aggression in the predatory sense. Inside a school, black skirts constantly test each other with short chases and mock lunges, and that rough-and-tumble is how their hierarchy settles. When the group is too small, there are not enough fellow skirts to absorb the attention, so the fish look further afield. Long, trailing fins make an irresistible target because they flutter like a rival's finnage during a chase. A black skirt is not trying to injure a betta; it is treating those fins the way it treats a schoolmate's, and the slow fish simply cannot get out of the way.
Tank mates that hold their own peacefully
Corydoras catfish are close to ideal: they are armoured, they live on the bottom where skirts rarely bother to follow, and they ignore the tetras completely. Similarly sized tetras with short fins, such as neons, rummy-nose and glowlights, share the middle water without offering anything worth chasing. Platies and mollies are confident livebearers with compact fins and do well, and a honey gourami makes a gentle centrepiece because its fins are short and its manner is unhurried without being sluggish. Nerite snails are left alone, and larger amano shrimp usually go unmolested, though baby shrimp small enough to swallow are at risk.
Combinations that end in shredded fins
Bettas are the classic mistake: slow, solitary and draped in exactly the finnage black skirts cannot resist. Guppies, especially the long-tailed fancy strains, fare little better, and angelfish combine veil-like fins with a stately pace that invites constant attention. Long-finned variants of other species, such as long-fin danios, carry the same risk even though their short-finned relatives are fine. Very small, timid fish are not nipped so much as crowded out at feeding time, so a black skirt tank is a poor place for shy nano species that need first access to food.
Stocking the tank so the peace holds
Give a school of six or more, and ideally eight, a long twenty-gallon tank at minimum so they have room to spread out and school properly. Keep the water between 75 and 82°F with a pH anywhere from 6.0 to 7.5, and plant the back and sides while leaving an open lane in the middle; stems of plants and pieces of wood break sight lines, so a chased fish can simply step out of view. Add the calmer residents first and the skirts last where you can, or rearrange the decor when newcomers arrive, so no fish is defending an established patch. For a pairing that still feels uncertain, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.