Coldwater Tanks: Unheated Aquariums Beyond Goldfish

Why Not Just Goldfish?

OATA, the UK ornamental fish trade body, notes that goldfish are often wrongly sold for small unheated aquariums. Goldfish grow large, produce a lot of waste and need big, well-filtered tanks or ponds. See goldfish tanks done right.

If you want a smaller unheated tank, choose fish that stay small and like cooler water.

How Cold Is Too Cold?

OATA says most tropical fish need at least 72°F (22°C), which means a heater. The fish on this page tolerate cooler water, down to about 63°F (17°C), so an unheated tank works in a home that stays warm all year. A tank in a cold room, a conservatory or near a drafty door may swing too far.

Put a thermometer in the tank and check it through the coldest and hottest weeks. If it drops below the range for your fish, add a heater set low. Keep the tank out of direct sun and away from radiators and drafts, which cause big swings.

Fish for Unheated Tanks

• White cloud mountain minnows: about 2 inches (5 cm), happiest on the cool side at about 63–73°F (17–23°C). Active; keep at least five with plants and a moderate current. See danios and white clouds.
• Zebra and leopard danios: about 2 inches (5 cm), very active, groups of at least five. Not every danio species can go unheated, so check.
• Rosy and golden barbs: up to about 4 inches (10 cm). Groups of at least five; some barbs nip fins, so avoid long-finned or shy tank mates.
• Variatus platies: about 2.4 to 3.1 inches (6 to 8 cm), calm and colorful. They breed readily, so keep single-sex groups or two females per male. See livebearers.
• Bronze and peppered corydoras: about 2.4 inches (6 cm), in groups of at least five on sand or fine gravel. See corydoras.
• Cherry and Amano shrimp: algae grazers that don't need a heater. Keep groups of five or more, and never use copper-based products; see shrimp and snails.

OATA's list is not complete, and your aquatics store may suggest other species. Always check that every fish you choose suits the same temperature range.

Setting Up

1. Choose the biggest tank you can fit; OATA suggests at least about 12 gallons (45 liters) for a small shoal of danios, white clouds or platies, and about 21 gallons (80 liters) for a corydoras shoal.
2. Add a filter: OATA stresses it is still essential without a heater.
3. Cycle the tank before adding fish; see cycling a tank.
4. Plant it well, leaving open water for active swimmers.
5. Add fish a few at a time and keep testing the water weekly.

Indoors Only

Cool-tolerant doesn't mean pond-safe. OATA notes that in Great Britain these species may only be kept in indoor aquariums, not outdoors, for biosecurity reasons. Many other countries also restrict keeping or releasing non-native fish. Never release aquarium fish or plants into ponds, rivers or lakes.

Rules differ by country. In Great Britain, see the gov.uk guidance on keeping non-native fish. In the US, check with your state wildlife agency; elsewhere, ask your national environment or fisheries authority.