The direct answer for corydoras in hot weather
A corydoras catfish rides out a heatwave on oxygen delivered to the bottom: extra aeration, a rippling surface, promptly removed waste, and slightly smaller meals until the weather turns. Corydoras live and feed on the substrate, and the bottom layer is where warm water loses its oxygen first — so while the tank's upper fish may look untroubled, the catfish school is breathing the thinnest water in the aquarium and needs the heatwave plan built around it.
Reading the surface trips correctly
A corydoras can gulp air at the surface and absorb it through its gut, and an occasional dash upward is ordinary behaviour in perfectly healthy water — owners new to the fish often panic at the first sight of it. The heatwave warning sign is frequency and company: one fish visiting now and then is normal, while the whole school queuing at the surface repeatedly, breathing fast between trips, or sitting motionless on the sand in a huddle means the bottom water is genuinely short of oxygen. Judge the school as a group, because corydoras take their confidence from numbers and a lone fish behaving oddly among relaxed tankmates is a different signal from a whole school struggling together.
Oxygenating the catfish's layer
Run an airstone so bubbles rise through the corydoras' usual resting area and set the filter outlet to keep the surface in constant gentle motion, since that surface exchange is where all the tank's oxygen enters. Shade the aquarium, cool the room where you can, and let evaporation work by using a mesh cover instead of a sealed lid during the hottest stretch. If the school still crowds upward, a modest water change with conditioned water a little cooler than the tank, poured in from a height to splash air in, gives the bottom layer a measured lift without a temperature swing.
Feeding and substrate care in the heat
Feed the corydoras slightly less during the hottest days and time their sinking food for the cooler evening, when the school is naturally most active. Siphon uneaten food and waste off the sand promptly — decaying matter on the substrate sits exactly where the catfish forage, and their whisker-like barbels are easily damaged by poor, dirty conditions, which then makes feeding itself harder for the fish. Keep the cleaning gentle and chemical-free; in a heatwave the answer to dirty sand is the siphon, never a harsh additive.
When the school needs outside help
Corydoras that keep gasping at the surface after aeration and a careful water change, barbels visibly eroded, or any fish rolling or unable to stay upright on the sand should be reported to an aquatic veterinarian or experienced fish shop the same day, with the tank's temperature and your changes so far described. Once the heat breaks, phase feeding back up and check the barbels and fins over the following week as the school recovers. Listing the aeration check, evening feed, and sand siphon on KinsLe Care Teams gives the corydoras' heatwave routine named owners instead of good intentions.