The short answer for pearl gouramis
Yes — pearl gouramis are among the most clearly tropical of the common community fish, and a heater is a firm requirement, not a judgment call. They come from some of the warmest freshwater habitats in the hobby and expect water in the upper 70s to low 80s Fahrenheit every day of the year. An unheated tank in most homes runs a good ten degrees below that outside of high summer, and a pearl gourami kept in it does not adapt; it retreats. You end up with a grey, hiding fish that gives no hint of the shimmering, orange-throated display animal the same individual becomes at the right temperature.
Swamp water and the labyrinth organ
Wild pearl gouramis drift through peat swamps, flooded forests, and sluggish lowland rivers in Malaysia, Indonesia, and Thailand, where the water is warm, soft, acidic, and often low in oxygen. Like other gouramis they carry a labyrinth organ, a maze of folded tissue above the gills that lets them breathe air from the surface — an adaptation that only makes sense in warm water, and one that ties their care to the air temperature as well as the water's. Gulping cold, dry room air can damage that organ, so a pearl gourami tank wants a close-fitting lid that traps a layer of warm, humid air at the surface. Their famous appearance is temperature-linked too: the constellation of pearly spots and the male's deep orange throat and chest are breeding-condition signals, produced by a fish whose whole system is running at tropical warmth.
Heating a tank for a large, long-lived fish
Set the heater for 77 to 82 degrees Fahrenheit and prove the reading with a thermometer away from the heater before the fish is stocked. Pearl gouramis grow to a substantial size and are typically kept in larger aquariums, which is quietly good news for heating: a big body of water changes temperature slowly, smoothing out the room's swings in a way a nano tank never can. Still, check the reading seasonally — a heater sized generously in summer may strain on the coldest winter night, and a fish this long-lived will see many winters with you. Run the heater during the cycle, watch it hold its range across a cold night, and only then consider the tank ready for a fish that may be with you for the better part of a decade.
Reading a pearl gourami's temperature complaints
A cold pearl gourami announces itself visually before anything else: the pearl spots dull, males lose their orange flush, and a normally bold, patrolling fish starts hovering behind plants and skipping feeds. Excess warmth flips the picture to restlessness and constant surface breathing. In either case, verify with the thermometer, adjust gradually, and let the fish settle at the corrected temperature for a few days before judging. If color and appetite do not return in proper warmth, or you notice frayed fins, bloating, or labored breathing that persists, that is the moment to bring in a veterinarian who treats fish — ideally with the fish's history in hand rather than reconstructed from memory. That history is exactly what a KinsLe Pet Passport holds together: where the fish came from, its tank parameters, and every change along the way, in one packet that travels to the vet, to a sitter, or to a new home without losing a page.