The short answer for a scarlet badis
A pair or trio of scarlet badis fits a 10-gallon tank, while a 20-gallon long is the right call for a group containing more than one male. The fish are tiny, with males wearing vivid scarlet striping, but they are perch-like hunters with a male's full sense of property. Each male picks a sheltered patch among plants or under a small cave, displays from it, and drives rivals away. Floor space divided by planting is what you are really buying with the larger tank: two or three defensible corners instead of one prize the whole tank fights over.
Why dense planting is part of the size calculation
Scarlet badis come from slow, vegetation-choked shallows in northern India, and they live by ambush, hovering motionless before picking off tiny invertebrates. A sparsely decorated tank, even a generously sized one, fails them because there are no hunting perches, no egg-guarding shelters, and no visual barriers. Females, plainer and rounder than males, need escape routes from courtship attention, and subordinate males need somewhere genuinely out of sight. Thick stems, moss, floating cover, small caves, and a scattering of leaf litter convert gallons into usable territory, so a planted 10-gallon honestly houses a trio while a bare 20-gallon may only house one contented male.
Shops often receive mostly males, since females are drab by comparison, so confirm the mix before planning a group around a tank size.
Feeding a shy hunter in its territory
Most scarlet badis ignore dry food and need small live or frozen items: tiny worms, daphnia-type foods, and freshly hatched brine shrimp offered near each fish's patch. In a community tank they are easily outcompeted and quietly starve, which is why a species tank or companions limited to very small, peaceful fish is the usual recommendation. Drop food into each territory rather than one central spot, and watch bellies rather than surface activity to judge whether everyone ate. Gentle filtration, subdued light under floating plants, and stable, clean water complete a setup that asks for patience more than equipment.
Males guard eggs laid in their shelter and colour up intensely while doing so, a display you will only see if the tank gives a male a territory he judges worth defending.
Signs the territories have run out
A male living behind the filter, torn fins, one fish colouring brilliantly while another stays grey and thin, or a female with nowhere to rest all mean the footprint or planting has fallen short. Add visual barriers first in a borderline tank, and move up a size when adding males rather than hoping they will share. Kept with a patch of jungle each, scarlet badis reward close watching like few nano fish. Mapping territories and noting which corner gets which food in KinsLe Family Sharing stops a covering helper from feeding only the bold male at the front glass.