How a school shows you it is too hot
A harlequin rasbora never suffers alone: as a true schooling fish it reads the group's behaviour, so heat stress shows as a school-wide change. The tight, glinting shoal loosens, fish drift up toward the surface or queue in the current under the filter outflow where oxygen is richest, and the coppery colour washes pale. Warm water holds less oxygen, and a school of active rasboras burns through what is there quickly, especially in the soft, warm conditions they prefer. Treat that behaviour as your alarm — by the time individual fish are gasping at the surface film, the whole school has been uncomfortable for hours.
Air and surface movement come first
Give the school oxygen before you chase degrees. Raise the filter outflow so it churns the surface, or add an air stone at the end of the tank away from the filter so circulation reaches the whole length. Rasboras come from slow, shaded forest streams, so aim for a rippled surface with a calm zone behind plants where the school can rest out of the current. Open the lid or rest it ajar on spacers to let hot air escape, and switch the light off during the peak of the day — rasboras are shade fish and will not miss it, while the tank loses a real source of heat. A fan played across the surface cools by evaporation; top the level back up each evening so the change in water level does not alter the filter's flow pattern the school relies on.
Gentle cooling, never a cold shock
If the temperature keeps climbing, change a modest portion of the water for dechlorinated water a degree or two cooler, poured in slowly at the filter end so it mixes before it reaches the school. Rasboras are sensitive to sudden chemistry and temperature shifts — big, cold water changes meant kindly can stress them more than the heat. Skip ice and frozen bottles except in a genuine emergency spike, and then float one briefly while you watch the thermometer and the fish, removing it as soon as the reading starts to move. Check the heater is off and healthy; a stuck heater is the one cause of a hot tank that no amount of fanning will beat.
Feeding a school in hot water
A feeding school is a frenzy, and in warm water every missed flake decays fast. Feed half portions twice rather than one full meal, and only while the fish are actively rising to eat — rasboras that hang back from a feed are telling you the water, not the menu, is the problem. Miss a day entirely if the temperature peaks; a healthy school carries plenty of reserve, and clean warm water beats fed foul water. Keep up small water changes through the hot spell, since a school's combined waste load in oxygen-thin water is the quiet risk behind most heatwave losses.
One routine the whole household follows
Heatwaves are when tanks get fed twice by two different people and lids get left shut by a third. Write the hot-day routine down where it is shared: who reads the thermometer at noon, who draws the shade, whether today's feed has happened. Family Sharing on KinsLe means a relative you pick can be given access to their profile, so the noon light-dimming can start with that profile open and the tank lid left shut.