The short answer for rice fish at home
Rice fish, often sold as medaka, are small, hardy schoolers, but small does not mean a desktop bowl. A group needs at least a 10-gallon tank, and a 20-gallon long aquarium is where they really shine, giving the school an unbroken swimming lane and enough water volume to forgive the wobbles of a busy household tank. These fish are active from morning to evening, feeding near the surface, displaying to each other in the open, and retiring into plant cover between bursts of activity. Plan for length first, a group of six or more second, and decoration last.
A fish built for open water and edges
In rice paddies and slow streams, medaka live where open, sunlit water meets dense vegetation, and a home tank should offer both zones. The open middle is where the school travels and where males show their colours; the planted edges are where fish rest, where females head when carrying eggs, and where fry first hide. A tank that is all plants chokes the swimming lane, while a bare tank leaves shy fish with nowhere to decompress. Aim for planted back and side margins with a clear central run, and keep some floating cover overhead so surface feeders feel sheltered rather than exposed. Gentle flow suits them; a torrent in a small tank exhausts fish this light-bodied.
Indoor tank, tub, or sheltered outdoor spot
Rice fish tolerate cooler water than most tropical schoolers, which opens options other fish do not have, from an unheated indoor tank in a bright room to a planted patio tub in a mild, shaded spot. Whatever the container, the space rules stay the same: enough length to swim, enough volume to stay stable, and a lid or cover because startled rice fish jump cleanly and accurately. An indoor aquarium still needs a proper light cycle and filtration or heavy planting to handle waste, and any outdoor setup needs shade from harsh midday sun and protection from visiting wildlife. Choose the setup your household can actually monitor daily, because a modest tank you watch beats a big tub you forget. Rice fish also spawn readily in good space, scattering eggs among fine plants and moss, so a roomy, planted tank quietly doubles as the fry's first nursery when parents are well fed.
Stocking and watching the school settle
Six rice fish is a workable minimum school, and eight to twelve in a 20-gallon long will school, scatter, and regroup in the way that makes this species so relaxing to watch. Suitable companions are other small, peaceful fish that share their cool, calm water; anything large, fin-nipping, or fast at feeding will shrink the medaka's world to the plant cover. In the first fortnight, look for fish using the whole tank: feeding at the surface, cruising the centre, resting in the edges. A school that hugs one corner is telling you the flow is too strong, the light too bare, or a tank mate too pushy. When several people help with this tank, KinsLe's Family Sharing puts the school count, tank size, and seasonal care notes into one profile the whole family can check.