The short answer for zebra danios
No, in most homes — and this answer surprises people, because the zebra danio is sold beside tropical fish as if it were one. It is not. The zebra danio is a subtropical fish from cool, oxygen-rich streams, and it is at its best in water that many tropical species would find uncomfortably cold. An unheated aquarium in a lived-in house typically sits between the mid 60s and mid 70s Fahrenheit, which is almost exactly the danio's home territory. Reaching for a heater out of habit can actually push a danio-only tank warmer than the fish prefer, shortening their active, zippy prime.
Built for cool mountain streams
Zebra danios come from streams in northern India and Nepal, in the foothill country below the Himalayas, where the water is cool, clear, and constantly moving. Their whole design reflects it: a torpedo body for holding station in current, a high activity level, and a metabolism that runs happily at temperatures that would shut a gourami down. This is also why danios have been used for generations as hardy first fish — they tolerate the cool, variable conditions of a new tank in a way genuinely tropical fish cannot. Keeping them well means respecting that cool-water nature rather than correcting it: brisk flow, plenty of swimming length, and water that stays on the cooler side of room temperature.
When the answer flips to yes
Two situations justify a heater. The first is a cold house: if the room regularly falls into the low 60s or below at night — a basement tank, a drafty old house in winter — a small heater set simply to take the edge off, holding the mid 60s, keeps the fish in range without cooking them. The second is company: danios are often mixed into tropical community tanks, and if their tankmates need 76 degrees, the danios will tolerate that warmth even though it is above their ideal — in that case the heater belongs to the tankmates, not to them. Either way, keep a thermometer in the tank and glance at it weekly. The danio's tolerance is wide, but it is not infinite, and an unheated tank in an unheated room during a cold snap can fall below even a danio's comfort.
Reading danios in cool water
Here is the pleasant part: in their preferred cool range, zebra danios do not show distress at all — they show off. A school in the high 60s is a blur of chasing, flashing stripes, and dawn spawning runs through the plants, and feeding time is a riot. Genuinely cold water, down in the 50s, finally slows them, and genuinely warm water makes them hyperactive in a way that burns them out early. If a danio school goes quiet, check the thermometer for extremes in both directions, not just the cold one. Persistent lethargy, wasting, or deaths in water that reads within their range points elsewhere — water quality first — and a veterinarian who sees fish or a good aquatics shop is the right call. When the call could go either way, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.