The short answer for a least killifish
A least killifish can be kept in a 5-gallon tank, yet a 10-gallon aquarium is the practical recommendation for most homes. This is one of the smallest livebearers in the hobby, a North American fish whose females are still only about the length of a thumbnail. The catch is not the size of one fish but the arithmetic of a colony: females release a few fry at a time over many days rather than in one large batch, so a group of six becomes a group of twenty without any single dramatic event. The 10-gallon gives that slow growth somewhere to go.
Why the name misleads, and why plants set the size
Least killifish are not true killifish in behaviour; they are livebearers related to guppies and mollies, grazing and picking around plants all day. Adults pose little threat to their own young compared with many livebearers, especially when the tank is thick with cover, which is exactly why fry survive and numbers build. Fine-leaved stems, floating plants with trailing roots, and mossy clumps create the layered shelter where fry feed on tiny organisms and females rest between drops.
In a bare 5-gallon those same fish look exposed, males pester females with nowhere to break the line of sight, and water conditions swing faster because there is so little volume buffering waste from a rising colony. The larger tank is not luxury; it is population and stability insurance.
Setting up a 5- to 10-gallon least killifish home
Use a sponge filter or cover any intake with fine foam, because newborn fry are small enough to be drawn in. Keep flow gentle, light moderate, and plant from the start rather than adding cover after fry appear. A small group of mixed sexes will establish itself, and surplus young can be moved on once you can net them without tearing the planting apart. Choose tankmates with extreme care or keep a species-only colony: most community fish large enough to notice the adults will happily eat the fry, and even the tiny adults can be outcompeted at feeding time.
Warm room temperatures suit them for much of the year and they tolerate a broad range, which makes them popular in unheated planted tanks, but a small heater guards against cold snaps in a small volume. Feed finely crushed foods and small live or frozen options in modest amounts spread across the plants, so shy fish and fry find some away from the boldest adults.
Watching a colony outgrow its enclosure
Count the fish monthly, including the fry hovering under floating roots, and treat steady growth as a prompt to thin the colony or upgrade before water quality tells you first. Males displaying constantly with no retreat for females, fry with no plant cover, and rising debris in a densely stocked tank all mean the enclosure has fallen behind the population. Kept ahead of that curve, this is a calm, self-sustaining little colony. Recording headcounts, plant trims, and who took surplus fry in KinsLe Family Sharing means the whole household sees the colony plan in the same shared profile, not just the person who set the tank up.