The short answer
The best true cold-water fish are white cloud mountain minnows, medaka (Japanese ricefish) and goldfish — species that thrive at normal room temperature without a heater. White clouds and medaka suit small unheated tanks; goldfish are cold-water too, but only for large, well-filtered setups because of their size and waste.
True cold-water species
- White cloud mountain minnow — the classic unheated-tank fish. Small, peaceful, colourful and happy anywhere from about 15–22 °C.
- Medaka ricefish — hardy, surface-loving little fish that tolerate cool water and even pond life.
- Goldfish — genuinely cold-water, but large, long-lived and messy. Give them space and heavy filtration, not a bowl.
Most popular aquarium fish — guppies, tetras, bettas — are tropical and do need a heater. Cold-water keeping means choosing species adapted to cooler temperatures rather than just leaving the heater off.
What “cold water” really means
A room-temperature tank still needs to be stable. Rooms swing warmer in summer and colder in winter, and rapid changes stress fish more than a steady cool temperature does. A bigger volume of water resists these swings, so err towards a larger tank. Watch out for placing the tank near radiators, sunny windows or draughty doors.
Keeping a cold-water tank healthy
The rules are the same as any tank: cycle it before adding fish, filter it well, and change 25–30% of the water weekly. Because cold water holds oxygen well, gentle surface movement from the filter is usually plenty.
Ready to plan one? Browse aquariums and start with the biggest you can fit — see how do I choose the right aquarium size. For hardy, forgiving stock, the answer to what is the hardiest aquarium fish overlaps closely with this list.