Cold-Water Fish

Cold-water fish are hardy freshwater species adapted to moderate water temperatures. They require sufficient water volume, good oxygenation and stable water conditions to thrive. These fish do not require an aquarium heater, but the aquarium should be kept in a temperate room and protected from excessive summer heat, as warmer water reduces the amount of dissolved oxygen available.

From goldfish and fancy goldfish to active schooling species such as Notropis, cold-water aquarium fish have different requirements in terms of adult size, swimming space and care.

Popular cold-water fish species

• Goldfish, Sarasa and Shubunkin: These hardy cold-water fish can reach 25–30 cm and are particularly well suited to ponds or very large aquariums. In an aquarium, their growth is strongly influenced by the amount of space available. As a general care guideline, allow at least 50 litres per fish to reduce the risk of stunted growth and help maintain stable water quality. Active and hardy, they appreciate plenty of open swimming space.
• Oranda, Ryukin and Japanese Fantail: These fancy goldfish varieties have stockier bodies and often long, delicate fins. In an aquarium, they generally reach around 15–20 cm. Because they are slower swimmers, they should be kept with calm, peaceful tank mates to reduce stress and prevent fin injuries. A spacious aquarium and stable water conditions are essential for their well-being.
• Notropis chrosomus: This small fish reaches around 5–8 cm and is particularly striking thanks to its blue iridescent coloration. Highly active and social, Notropis chrosomus should be kept in a school of at least five individuals. Regular water movement and sufficient oxygenation allow it to display its natural behaviour.
• Notropis lutipinnis: This active species requires a long aquarium with plenty of space for continuous swimming. This freshwater cyprinid reaches approximately 7–8 cm and should also be kept in a group. It is well suited to well-oxygenated cold-water conditions, and its social nature means it should be kept in a school to encourage balanced, natural behaviour.
• Myxocyprinus asiaticus: This peaceful and curious fish becomes very large as an adult, reaching approximately 60 cm. It is therefore better suited to ponds or very large aquariums. Herbivorous, it grazes on algae and soft vegetables. It appreciates a safe environment with rounded rocks, large hiding areas and cool, well-oxygenated water.

Cold-water fish diet and behaviour

The metabolism of cold-water fish is directly influenced by water temperature. Most species are omnivorous and benefit from a varied diet.

For stocky fancy goldfish, sinking pellets are recommended to help reduce the amount of air swallowed while feeding, a common cause of swim bladder problems. Their diet can also be supplemented with frozen foods such as Artemia (brine shrimp) and Daphnia to support vitality and enhance their colours.

Cold-water fish are naturally social and active. Swimming and exploring their surroundings are characteristic behaviours that contribute to their well-being. Aquarium plants should be robust, such as Anubias and Java fern, as goldfish may nibble on or dislodge more delicate plants.

A well-structured aquarium combined with a suitable diet provides a stable and secure environment where these fish can display their natural behaviour.

Cold-water aquarium care

Successful cold-water aquarium care is based on three essential factors:

• Tank size and swimming space: An aquarium of around 200 litres provides a suitable starting point for a small group of 3 to 4 fancy goldfish. For Notropis, an aquarium of at least 80–100 litres provides enough space for comfortable swimming and allows them to display their natural behaviour.
• Filtration: Cold-water fish produce a significant amount of waste. The aquarium filtration system should be capable of processing approximately 3 to 5 times the aquarium volume per hour to help maintain good water quality.
• Water quality: A partial water change of 10–20% each week is recommended, while the pH should remain neutral to slightly alkaline, at around 7–8. During summer, if the water temperature rises above 25°C, an air pump or surface fan can help maintain sufficient oxygen levels.

The cold-water aquarium setup should include driftwood and smooth, non-sharp rocks, providing resting areas while leaving plenty of open space in the centre for swimming.

A stable aquarium environment and a suitable diet are essential for supporting the health, well-being and development of cold-water fish.