The short answer for mystery snails
Sometimes — and this snail is the honest exception among tropical tank residents. Mystery snails tolerate a broader temperature span than most fish they live with, so the right answer depends less on the species and more on your house. A tank in a warm room that holds steady in the low-to-mid 70s Fahrenheit needs no heater at all. A tank in a cool room, a basement, or a spot that swings between day and night benefits from a small heater, not because the snail will die without one, but because its whole pace of life is set by the water temperature around it.
Temperature is a mystery snail's throttle
A mystery snail is cold-blooded in the plainest sense: warm it and everything speeds up, cool it and everything slows. In water in the upper 70s, a mystery snail is a busy animal — gliding over glass and plants, extending its siphon to taste the surface, eating generously, and laying down new shell at the growing edge. Drop the water into the 60s and the same snail draws into its shell, seals the door, and waits, sometimes for days, in a torpor that worries new owners who think it has died. Neither state is an emergency by itself, but the warm, active snail is the one building a strong shell and living fully, while the cooled snail is merely enduring. Warmth also interacts with appetite: a snail that barely eats in cold water takes in less of the calcium-bearing food its shell depends on.
Deciding for your tank, and heating it safely
Check your actual tank temperature across a full day and a cold night before deciding — many owners are surprised by how cool an unheated tank runs after dark. If readings sit in the low 70s or above and barely move, skip the heater with a clear conscience. If they sag into the 60s or swing widely, add a modest heater set for the mid-to-upper 70s and confirm with a thermometer. Place the heater where a grazing snail is unlikely to park itself against the hot surface, and never let a water change refill the tank with water far colder than what is in it; a sudden cold plunge can shock a warm, active snail into sealing up and not emerging.
Reading the snail in front of you
Learn to read behavior rather than guessing from the calendar. A comfortable mystery snail cruises daily, its shell's growing edge staying smooth and even. A snail floating for a day can simply be resting with air in its shell, but one that stays sealed and unresponsive for many days in cool water is most likely torpid — warm the tank gradually and it should emerge. A dead snail, by contrast, hangs limp from the shell and smells unmistakably bad, and should be removed at once. If a snail's shell erodes or its appetite stays poor despite proper warmth and varied food, that is the time to ask a veterinarian familiar with invertebrates or a good aquatics shop for a second look. Their care is simple but easy to let slide, and KinsLe's Care Teams turns it into small assigned duties — a feeding day here, a shell check there — each marked done with a timestamp so the snail's routine survives busy weeks.