What is the absolute minimum tank size for a Least killifish?

5 US gallons as an adult, no strict footprint requirement. Seriously Fish Heterandria formosa profile: a pair fits 30 × 20 × 20 cm (12.5 L); a small colony is best.

Can I keep a Least killifish in a smaller tank when it's a juvenile?

Briefly, yes — fish stores often sell juveniles that haven't reached adult size yet. But you should plan, buy, and cycle the adult-minimum tank from day one. Upgrading mid-life stresses the fish and is far more expensive than buying the right tank up front.

Can I keep more than one Least killifish together?

It depends on the species' behaviour (peaceful). Least killifish can usually be kept in small groups in an appropriately sized tank.

What temperature and pH does a Least killifish need?

Temperature 68–78 °F, pH 7–8, hardness 5–20 dGH.

Tank size · Freshwater

What size tank for a Least killifish? (5 gal minimum)

Least killifish (Heterandria formosa) — adult 1.2″, slim-bodied, peaceful.

The three sizes — minimum, good, ideal

Tier	Tank size	What it gets you
Absolute minimum	5 US gal	Survives adult life. No extras — no tankmates beyond the essentials, no aquascape that eats footprint.
Good	8 US gal	Room for a small community, decor, and parameter buffering. This is where most people should aim.
Ideal	13 US gal	Natural-feeling space. Active swimmers stop pacing, schoolers school freely, aggression drops.

Parameters at a glance

Temperature: 68–78 °F
pH: 7–8
Hardness: 5–20 dGH
Adult size: 1.2″
Body type: slim

Source for the tank-size minimum

Seriously Fish Heterandria formosa profile: a pair fits 30 × 20 × 20 cm (12.5 L); a small colony is best.

Next steps

Build a candidate stocking list with the aquarium stocking calculator — it'll warn if your community is incompatible.
Confirm tank water volume after substrate and rock with the aquarium volume calculator.
Read the nitrogen-cycle guide before adding the first fish.