The short answer for dwarf gouramis
Yes. Of all the common community fish, the dwarf gourami is one of the least suited to an unheated tank. This is a fish built for warm, still, heavily planted water, and it asks for a reliably heated aquarium in the upper 70s to low 80s Fahrenheit. An unheated tank in a typical house sits ten degrees below that for much of the year, and a dwarf gourami kept cold does not merely slow down — it becomes a sitting target for the bacterial and viral problems this species is unfortunately known for. The heater is not an accessory here; it is the foundation the rest of the setup stands on.
Why warmth matters so much to this fish
Dwarf gouramis come from slow rivers, ponds, and flooded fields in India, Bangladesh, and Pakistan, where dense vegetation stills the current and the sun keeps shallow water warm. They are labyrinth fish: above their gills sits a folded organ that lets them breathe air at the surface, an adaptation to warm, oxygen-poor water. That biology explains two care points at once. Their metabolism and immune system expect tropical warmth, and the air they gulp should be warm and humid too — a cold draft across an open tank top can genuinely harm them, which is why a close-fitting lid matters for this species more than most.
Setting the temperature, and keeping it steady
Set the heater for roughly 77 to 82 degrees Fahrenheit and verify it with a thermometer placed away from the heater itself. Run the heater during the cycle, before the fish arrives, and watch the reading across a full day and night; a tank that holds steady while the room cools after dark is ready. Because dwarf gouramis are often kept singly or as one male with females in a modest tank, remember that smaller volumes swing faster — check the temperature whenever the weather turns, and treat a heater that cannot hold the range on a cold night as one to replace, not to tolerate. Steady warmth will not guarantee a healthy gourami, but unsteady cool water nearly guarantees trouble.
Reading a gourami that is too cold
A cold dwarf gourami fades: the powder-blue and orange-red bands wash out, the fish hangs motionless in a corner or rests on the bottom, and it loses interest in food it normally rushes for. It may also hover just under the surface gulping more often than usual. Correct the temperature gradually — a degree or two at a time — rather than swinging the heater to maximum, and if appetite and color do not return within a couple of days of proper warmth, or you see bloating, sores, or stringy pale waste, that is the point to involve a veterinarian who treats fish, history in hand. Keeping that history is easier than it sounds: a KinsLe Pet Passport gathers your gourami's records — purchase date, tank parameters, treatments and changes — into one portable packet, ready for a vet visit or a handoff to a sitter.