What unheated really means for the fish
An unheated tank takes its temperature from the room, so the question is never whether the fish can survive without a heater in the abstract — it is what temperature your house actually holds, hour by hour, through the coldest night of the year. A warm house holding steady somewhere in the seventies is, from a fish's point of view, a mild subtropical climate, and a whole range of popular species evolved in exactly that kind of water. What fish cannot handle is not coolness itself but swings: a tank that drifts down sharply overnight and climbs again by afternoon stresses even hardy species, which is why the size and placement of the tank matter as much as the species list.
Fish that genuinely suit room-temperature water
White cloud mountain minnows come from cool Chinese streams and are at their best unheated, colouring up strongly in water that would chill a tetra. Zebra danios share that tolerance and add constant motion. Guppies and platies are comfortable through the low seventies and breed happily there, though their colours and growth peak a touch warmer. Medaka ricefish are famously unfussy, raised outdoors in Japan through real seasons. Paradise fish, one of the oldest aquarium species, prefer room temperature and sulk in hot water. Cherry barbs, most corydoras of the hardier kinds, and variatus platies round out a list that can furnish a complete community without a heater anywhere in it.
Stability matters more than the heater you skipped
Position is your temperature control. Keep the tank on an interior wall, away from exterior doors, radiators and windows that bake it in afternoon sun, and fit a simple thermometer so the daily range is something you know rather than assume. Larger volumes buffer change — a twenty-gallon tank cools slowly through a cold night where a five-gallon can drop several degrees before morning. A lid helps twice, slowing evaporation and holding a cushion of warm air above the surface. If your house uses night setback heating or air conditioning that creates real swings, those swings, not the average temperature, decide which species are fair choices.
When an unheated setup stops being fair
There are limits worth respecting. True tropical fish such as discus, rams, most gouramis and cardinal tetras need stable warmth an unheated room cannot promise, and keeping them cool invites the white spot outbreaks and listlessness that follow chronic chilling. Houses that sit empty and cold through winter holidays, rooms heated only when occupied, and small tanks on windowsills all push room-temperature keeping past what the hardy species should be asked to handle. A small heater kept purely as insurance, set just below your room's normal low, costs little and only ever switches on during the cold snap you did not predict.
Check the range before the fish comes home
The commonest unheated-tank mistake is trusting the category on the shop label, where temperate species get shelved with tropicals and the temperature ranges on tickets are optimistic at best. A minute spent checking the real range for each species you fancy — and for whether it mixes with the others on your list at that temperature — prevents the slow, confusing losses that make people give up on heater-free keeping. For a stocking list built at the shop counter, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.