The short answer for a very shy fish
A scarlet badis is barely an inch long, brilliantly coloured, and famously retiring, and its home should be sized by cover rather than by gallons alone. A 10-gallon tank suits a pair or a small group when it is densely planted and full of small shelters, while a 20-gallon aquarium is the safer choice for several males. An open 20-gallon with two plastic plants offers this fish less real room than a thickly grown 10-gallon, because a badis experiences space as a chain of hiding places it can slip between, not as open water it will cross. Plant first, then count gallons.
Territories measured in leaves
A male scarlet badis claims a small patch of planting, courts females who wander through it, and chases rival males who cross its invisible borders. Females and juveniles keep looser ranges but still live close to cover, hunting tiny live foods among stems and leaf litter. That is why broken sight lines are the true space requirement: when each male can hold a patch he cannot see a rival from, squabbles stay brief and nobody lives under constant pressure. Thick stem plants, small caves, curled leaves, and a carpet of litter on the substrate all subdivide a modest tank into many private corners. Bare glass, by contrast, puts every fish on display to every other fish all day.
Building the thicket they need
Choose a tank with a decent footprint rather than a tall column, since badis live in the lower half among the planting. Lay a fine substrate, plant heavily from the back to the midground, and tuck several small caves or dense clumps at ground level spaced along the length, roughly one strong shelter per fish. Keep flow very gentle, because these fish come from slow, vegetation-choked water and a strong filter current flattens both the plants and their willingness to venture out. Add a lid for safety during maintenance. Because badis are micropredators that hunt by watching and stalking, scatter their tiny foods through different thickets on different days, so every fish gets hunting time inside its own patch rather than gathering nervously at one feeding point. Feed small live or frozen foods into the planting rather than one open spot, so shy fish can eat inside their own patches instead of competing under a spotlight.
Reading whether the space is working
A well-arranged badis tank looks quiet at first glance and busy on a patient watch: males glow red from their patches, females slip through the stems, and everybody hunts. Warning signs are specific. A male who never colours up, a fish permanently exiled to a top corner, or constant flaring through the plants means territories are colliding and the layout needs more subdivision or the group needs more room. Because badis are slow, deliberate feeders, pushy tank mates shrink their world too; keep them with equally small, calm companions or alone as a species group. When more than one person tends this delicate setup, KinsLe's Family Sharing keeps the group makeup, shelter layout, and feeding notes in a single profile everyone can follow.