Twenty gallons is really about the school
Asking what tank a black skirt tetra needs is really asking what tank six of them need, because this species should never live alone. Twenty gallons is the practical floor: it gives a school of six deep-bodied, two-inch tetras room to spread through the middle level, hold their small territories and still regroup as a school. A twenty-nine-gallon tank is kinder still, letting you keep eight to ten and add a bottom level of corydoras without crowding.
Shape matters as much as volume. The long twenty-gallon, with its thirty-inch run, suits black skirts far better than a tall tank of the same volume, because these fish travel horizontally and barely use the upper and lower thirds. Height is wasted on them; length is welfare.
The fish you are housing
Black skirts — sold young with dramatic black rear fins that fade to grey as they mature — are active, long-lived tetras that can be with you for years. Juveniles in the shop are a fraction of their adult depth, and planning the tank for the adult school avoids the common arc of a fish that quietly outgrows a ten-gallon starter. In a settled school there is mild sparring, especially among males, and fins stay intact when each fish can yield ground — another thing only space provides.
Warm water in the mid-to-upper seventies, neutral-ish chemistry and subdued planting at the back with an open centre match their flooded-forest origins.
Layout and tank mates
Give the school a front swimming lane, plants and wood along the back for retreat, and a lid, since a startled tetra can jump. Tank mates should share their pace: other mid-sized tetras, peaceful barbs, danios and bottom catfish all work. The long-finned white skirt colour form of this same species, and other slow fish with flowing fins, are poor companions, because black skirts investigate trailing fins — a habit that fades in big schools with room, and worsens in cramped ones.
Feeding is easy — they take most small foods eagerly — but scatter it so the school's boldest members do not monopolise the surface while the shy ones hang back.
The black skirt's two colour forms deserve a sizing footnote. The natural form greys with age while the white skirt form stays pale, and shops often display them in separate tanks, but they are one species and school together perfectly well. A mixed group of six across both forms behaves as a single school and looks striking doing it, though the white form's longer fins deserve the roomier end of the sizing advice, since flowing fins plus a tight tank is exactly the combination that invites nipping.
Growing room and record-keeping
If you begin with six juveniles in the minimum tank, the honest plan includes either staying at six or upgrading before adding. When you do expand the school, add at least three at a time and rearrange a little decor so old territories dissolve. It helps to keep a plain record of the setup — tank size, school count, dates fish were added, filter and heater details — because that is the information a shop, a sitter or a future owner actually asks for. KinsLe Pet Passport gathers that setup history into one portable packet for the black skirts, ready for a vet visit, a handover or a house move.