The short answer for rice fish
A group of rice fish starts comfortably in a 10-gallon tank, and a 20-gallon long is the better long-term home for a school that will almost certainly breed. Rice fish, the medaka long kept in Japan in bowls, tubs, and rice paddies, are small, peaceful, and unusually hardy, tolerating a wide temperature range that includes unheated indoor tanks. Their needs are simple but specific: company of their own kind, surface area for their active top-level swimming, and planted corners where eggs and fry can survive. Depth contributes little; a long, open footprint contributes a lot.
Why surface and school size drive the choice
Rice fish spend much of their time in the upper third, cruising and displaying in loose formation, with males showing brighter colour as they court. Females carry clusters of eggs on their bodies before depositing them on fine plants or spawning mops. In a group of six to eight or more that behaviour runs all season in warm conditions, and fry appear in the plants without any special effort. Stock a tank too small and the school crowds the surface, dominant males monopolise the spawning plants, and the fry crop is eaten because there is nowhere out of the adults' patrol route.
The 20-gallon long solves this by geometry: more surface to cruise, separate plant clumps at each end, and a quieter corner where fry can grow before joining the school. It also steadies an unheated tank against room temperature swings far better than a small volume does.
Indoor tanks, outdoor tubs, and the in-between
The same footprint logic applies outdoors, where rice fish are often kept in patio tubs and mini ponds viewed from above, which shows off the pearl and orange colour forms beautifully. A tub needs shade for part of the day, plants for cover, and a mesh cover where local wildlife hunts; bring fish indoors or deepen their shelter where winters freeze hard. Indoors, gentle filtration or even a well-planted low-tech setup suits them, with a lid or lowered waterline because spawning excitement produces jumpers.
Feed small floating and crushed foods at the surface, and leave fine-leaved plants or mops in place during breeding season if you want fry. Tankmates should be equally small and peaceful, or the rice fish kept as a species group, which most keepers end up preferring.
Knowing when the school has outgrown the tank
Because breeding is so reliable, the honest sizing question is not the six fish you bought but the twenty you may own by autumn. Surface crowding, fry that never survive past a few days, and males in constant contact are your cues to thin the group, move some to a tub, or step up a tank size. Managed that way, rice fish are among the most rewarding small fish in the hobby. If your group splits its year between an indoor tank and a summer tub, KinsLe Family Sharing keeps both locations and the moving dates in one shared profile so nobody feeds an empty tank.