The direct answer for a bottom-dweller in the heat
A bristlenose pleco's heatwave care is about the bottom of the tank: put extra aeration low, keep the surface moving, trim feeding, and judge the fish at night when it is honestly active. Warm water holds less dissolved oxygen, and the layer that depletes first is the still water along the substrate — precisely where a pleco lives, grazes, and sleeps — so a tank that looks fine to its surface fish can be quietly suffocating its pleco.
Why heat lands hardest on this fish
A bristlenose pleco produces a generous amount of waste for its size, and in a heatwave that waste decays faster, consuming oxygen and releasing ammonia into the bottom layer the fish cannot leave. At the same time the pleco's own metabolism speeds up with the temperature, raising its oxygen needs while supply falls. Because the fish is nocturnal and secretive, the early signs are easy to miss: a pleco that usually emerges at dusk stays in its cave, or makes repeated trips to the surface and upper glass in the dark — behaviour you only catch if you look after lights-out.
Aerating the bottom layer
Place an airstone so its bubbles rise from near the bristlenose pleco's grazing area, and angle the filter outlet to keep the whole surface rippling, since surface movement is what loads oxygen into the water for the airstone to distribute. If the pleco shares the tank with surface fish that look comfortable, do not let that reassure you about the bottom — aerate for the pleco's layer, not the top. Keep the tank shaded and the room as cool as you reasonably can, and resist reorganising caves and wood to 'check' the fish; its shaded driftwood is its coolest refuge and it should stay exactly where it is.
Feeding and cleaning while temperatures stay high
Offer the bristlenose pleco slightly smaller portions of its algae wafers and vegetables during the heat, and lift out uneaten pieces sooner than usual — a softening vegetable slice forgotten behind wood is an ammonia source sitting in the fish's bedroom. Vacuum the gravel or sand around the feeding area during a modest water change with temperature-matched, conditioned water, because the waste concentrated there is the heatwave's main chemical threat. Keep changes modest and steady rather than one dramatic overhaul in hot weather.
Night checks and the point of concern
Check the bristlenose pleco after dark with a dim torch: normal heatwave behaviour is grazing and wood-rasping with a slightly busier pace, while a fish hanging at the surface, breathing rapidly on the glass near the filter outlet, or lying unresponsive in the open needs the aeration increased immediately and advice from an aquatic veterinarian or experienced fish shop if it does not settle. Note the tank's peak temperature and what you changed each day. Writing those heatwave notes into the pleco's KinsLe Pet Passport keeps its temperatures, water changes, and vet conversations together in one packet that can go straight to an appointment.