Cool-water fish, but not a freezing-water fish
A white cloud mountain minnow needs protection from extremes during very cold winter weather, even though it is one of the hardier cool-water aquarium fish. White clouds come from cool mountain streams and happily live in water that would chill a scarlet badis or a betta, so many keepers run their tank without a heater for much of the year. Very cold weather is different. A tank in a porch, garage, or unheated spare room can drop far below the cool range these minnows enjoy, and a white cloud mountain minnow that is near-freezing will stop feeding, lose its red fin colour, and sit listlessly near the bottom.
The mistake to avoid: sudden overheating
Sudden overheating is the winter mistake most specific to a white cloud mountain minnow. A keeper who worries during a cold snap sometimes adds a powerful heater set for tropical fish, and the minnows then face water far warmer than they prefer, with less dissolved oxygen and faster bacterial growth in uneaten food. If a white cloud tank does need protection, use a small, adjustable heater only to take the edge off the cold and prevent a plunge, and raise the temperature gradually over many hours. A thermometer matters here for the opposite reason to a tropical tank: it proves the water has not been pushed too warm as well as too cold.
Placement, flow, and winter feeding
Place a white cloud mountain minnow tank inside the lived-in part of the home for the coldest weeks if its usual room is unheated. Keep the gentle flow these fish enjoy from their stream habitat, since a filter switched off to save heat leaves waste to build up in still, cold water. Insulate the back of the tank, keep the lid on to slow heat loss and stop a startled minnow jumping, and top up evaporation with water of a similar cool temperature rather than warm tap water. Feed smaller amounts in winter; white clouds in cool water eat eagerly on some days and ignore food on colder ones, and leftover flakes decay even in a cool tank. Clear uneaten food after a few minutes.
When cool becomes too cold
Treat a white cloud mountain minnow that has stopped schooling, turned pale, or stayed at the bottom through a full day as too cold or unwell, not simply dormant. Check the thermometer reading against the range the fish are used to, confirm the filter is running, and test the water before warming anything. Warm the tank slowly by moving it to a warmer room or using that low, guarded heating, never by pouring in warm water. If minnows gasp at the surface after warming, or several die suddenly during a cold spell, contact an aquatic veterinarian or trusted aquatic shop for advice. Also think about where winter water for changes comes from, because a white cloud tank topped up from an outdoor butt or a very cold tap can drop several degrees in minutes. Bring change water indoors the day before, let it reach the tank's own cool temperature, and add it slowly at the filter outlet so the minnows are not hit by a cold stream. Recording the exact setup prevents well-meant mistakes by other helpers, and KinsLe keeps that detail in Pet Passport, where the minnows' cooler range, foods, and filter notes travel together as one packet.