Size for a colony, not for a single mouse
Pet mice are almost always kept as a group of females, and their space should be planned for the colony. A 20-gallon long tank or an equal-footprint barred cage is a fair minimum for a small group, while a 40-gallon breeder tank or a long glass cabinet conversion makes a visibly better home — more room for separate nesting, feeding and wheel areas, and less tension at pinch points. Females live together happily in the right setup, whereas a male mouse usually lives alone and still deserves the same generous footprint, filled with extra enrichment because he has no cage mates to wrestle, groom and sleep against.
Mice want cover, not open floor
A big empty cage frightens mice more than a smaller, layered one. Aim for a home where a mouse can cross from end to end mostly under cover: deep bedding for tunnelling and nest building, cardboard tubes and cork pieces at ground level, then ropes, branches, ladders and hammocks using the height above. Add at least two solid-surface wheels around eight inches across for a group, several hides with small entrances, and scatter feeding through the bedding so the whole territory becomes a foraging ground. Bar spacing on a cage must be narrow — mice flow through gaps that would stop a hamster — which is one reason glass tanks with mesh lids suit them so well.
Nests, scent and how not to clean away the map
Mice navigate largely by scent and rebuild constantly. Give them shredded paper and hay to carry, expect an elaborate communal nest in one chosen corner, and clean in halves: wash toys and wheels weekly, but replace bedding in sections and return a handful of the old nesting material, so the colony's scent map survives the clean. Full strip-outs done too often are a common trigger for squabbles, because every mouse suddenly smells like a stranger in a strange place. Extra space helps here too — a larger home develops distinct zones, and a mouse can retreat to a quiet corner instead of defending the only hide.
Securing the lid and spacing the bars
Mice are escape artists of a high order, so the enclosure's security is part of its size: a tank lid must be fine mesh, weighted or clipped, with no flex a mouse can lever, and any barred cage needs spacing around a quarter of an inch for young mice. Check corners where tubes join and any hole drilled for a bottle spout, because a mouse who finds a gap will teach the whole colony where it is within the night. A secure, generous tank also lets you house the group through its whole life without moves — and mice, who organise their friendships around a shared nest and territory, are noticeably calmer for never being rehoused.
Keeping a whole colony straight as a family
A tank of near-identical mice defeats memory quickly: which one is the shy new rescue, who lost weight last week, which wheel squeaks. Recording the group properly at the start — names, markings and ages — makes daily care far more observant. Relatives you pick are given access to open the profile for this colony when you share it using KinsLe Family Sharing.