Coldwater Aquariums
A guide to unheated coldwater aquariums: which species suit room temperature, including goldfish, white cloud minnows and hillstream loaches.
Overview
A coldwater aquarium is one kept without a heater, relying on stable room temperature instead. It suits fish that originate from cool or temperate waters rather than tropical reefs and rivers. Because no heater is used, the room itself sets the temperature, so the tank should be placed away from heat vents and draughty doors.
Goldfish
Goldfish are classic coldwater fish that live well at 50-70 °F (10-21 °C), so a heater is unnecessary in a normal heated home. They are heavy eaters that produce a lot of waste and need strong filtration. A common stocking guide is about 20 gallons for the first goldfish plus at least 10 gallons for each additional fish, in a wide tank with good surface area.
White Cloud Mountain minnows
White Cloud Mountain minnows reach about 1.5 inches (4 cm) and live comfortably at 65-77 °F (18-25 °C) with a pH of 6.5-8.5, readily kept with no heater. They are schooling fish best kept in groups of at least six in a 10-gallon tank or larger and are notably tolerant of cool conditions.
Hillstream loaches
Hillstream loaches are cool-water fish from fast-flowing rivers that accept a range of about 65-80 °F (20-27 °C), though they are more prone to stress at the higher end. They value stable pH around 7.0-7.8 and good water quality, and as strong algae eaters they consume hair algae, brown diatoms and even black beard algae.
Other unheated options
Several other species tolerate unheated tanks because they come from cooler waters, including dojo loaches, certain barbs, rainbow shiners and cherry shrimp. Matching fish to the natural room temperature avoids the need for a heater.
Temperature stability
Even without a heater, large temperature swings are stressful. Keeping the tank in a room with a steady temperature, away from windows, radiators and electronics that throw off heat, gives coldwater fish the consistency they need.