Corydoras live where the water is coolest
Corydoras spend their lives on the substrate, and that simple fact changes the heater question. Water stratifies slightly in a still tank and the bottom, where these fish feed, breathe and rest in a group, is the coolest zone — so a tank that reads acceptable at the surface can run a degree or two lower exactly where the corydoras sit. Most bronze, peppered, panda and sterbai corydoras sold for community tanks come from warm South American water and do best somewhere between 22 and 26 degrees, with sterbai comfortable warmer still. In a home that holds the low twenties all winter, that range may need no help; in a cooler house, or one that drops at night, a heater is what keeps the bottom layer inside it instead of visiting the teens while you sleep.
Species differences worth respecting
Corydoras is a big family and the label on the shop tank decides the setting, not the genus. Peppered corydoras are famously tolerant of cooler water and can be content at the bottom of the range, while sterbai kept too cool go quiet and lose condition. That is why mixing species with different preferences in one tank, then heating to a compromise none of them chose, is a common low-grade mistake. Identify yours properly, set the heater for the species you actually have, and keep the group to one kind in numbers of six or more so behaviour stays natural. Temperature stability also interacts with their breeding triggers — a slightly cooler water change can prompt spawning in a conditioned group — which only works as a gentle nudge when the baseline the heater holds is warm and steady to begin with.
Placing heat for bottom dwellers
Put the heater where filter flow washes past it and carries warmth down and across the substrate, and check the temperature low in the tank near where the group rests rather than only at the surface strip thermometer. Sand matters to the whole picture: corydoras sift it through their gills and rest their barbels on it, and a fish kept warm but on sharp gravel still wears its barbels down, so treat temperature and substrate as one comfort package. Leave open sand in front of planting so the group can gather in view, and keep a lid gap-free — a startled cory can dart upward like any other fish. In summer, watch the reverse: a small tank in a warm room can push past the top of their range while the heater sits idle, and extra surface agitation is the gentle fix.
Reading the group for temperature trouble
A settled cory group shuffles, sifts and rests together, with occasional dashes to the surface for a gulp of air — that dashing is normal and not, by itself, distress. Worry when the dashes become constant across the whole group, when barbels look short or red, or when fish that normally feed in a scrum hang back listless on the sand. Test the water and check the low temperature first, since waste collects at their level too and ammonia near the substrate is a frequent partner to temperature drift. If several fish deteriorate or barbels erode despite clean, correctly warm water, seek an aquatic vet's input. Sharing the low-level thermometer reading as a named task in KinsLe via Care Teams means the check survives busy weeks and holidays.