The direct answer for a cool-water nano fish
A celestial pearl danio needs its tank moved to the coolest spot in the home, its lights rested during the hottest hours, and its temperature checked twice a day for the whole heatwave. Unlike many tropical fish that merely endure heat, this species comes from cool, spring-fed, heavily planted ponds and keeps its best colour and behaviour at the cooler end of aquarium ranges — sustained heat is a real danger to it, not a passing discomfort to wait out.
Why small fish in small tanks heat first
A celestial pearl danio usually lives in a nano aquarium, and a few litres of water follow room temperature within hours, offering no cool refuge layer at all. The fish themselves are tiny, shy, and plant-oriented: a hot celestial pearl danio stops displaying, fades from its spotted, fin-flaring best, and hides deep in the vegetation, which means the owner of an unattended nano tank can miss two days of decline behind the plants. Heat also thins the oxygen in exactly the small, still, plant-dense water this species prefers.
Relocating and shading the nano tank
Move the celestial pearl danios' aquarium — carefully, partially drained if it must travel far — to the coolest room available, away from windows, exterior walls that bake in the afternoon, and heat-producing appliances. If moving is impossible, shade the window, aim room airflow across the water's surface, and open the lid or use a mesh cover during the worst hours so evaporation can carry heat away, accepting a little extra top-up water in exchange. Set the aquarium light to run only in the cooler morning and evening, or rest it entirely on the hottest days; the plants will forgive a few dim days far more easily than the fish forgive hot ones.
Feeding and water during the hot spell
Feed the celestial pearl danios lightly once a day while the heat lasts, choosing foods they finish in moments, because decaying micro-food in a hot nano tank degrades the little water there is very quickly. Top up evaporation with conditioned water little and often rather than in one large pour, and if a water change is needed, make it small and use water a shade cooler, added slowly, so the temperature steps down instead of swinging. Keep hands and nets out of the plants — chasing a shy fish in hot water for a closer look spends oxygen it cannot spare.
Signs the danios need more than home cooling
A celestial pearl danio school that stays hidden through a cool morning, breathes rapidly at the surface film, or loses its colour entirely after the tank has been shaded and aerated needs prompt advice from an experienced fish shop or aquatic veterinarian, with the peak temperature you recorded ready to report. After the heatwave, restore the normal light and feeding schedule over a couple of days and watch the plants and water clarity as the tank's balance recovers. Posting the relocation, lighting, and feeding routine in KinsLe Family Sharing means any family member home during the day can follow the danios' heatwave plan exactly.