The short answer for a harlequin school
Yes. Harlequin rasboras are tropical fish through and through, and a heater belongs in their tank from day one. In most houses an unheated aquarium settles somewhere in the high 60s Fahrenheit, a few degrees below the range these fish are built for, and while a harlequin will survive a cool spell, it will not show you any of the behavior that makes the species worth keeping — the tight, flashing shoal moving as one body through the plants. Warmth, for this fish, is the difference between a school that merely exists and one that performs.
The peat swamps behind the requirement
Wild harlequins live in Malaysia, Singapore, and parts of Indonesia and Thailand, in slow blackwater streams and peat swamps stained tea-brown by fallen leaves. The water is soft, slightly acidic, shaded, and reliably warm, sitting in the mid-to-upper 70s. Their copper-orange bodies and bold black wedge evolved for that dim, warm world, and their digestion, activity level, and breeding condition all assume it. A school kept at the right temperature colors up within days of settling in; the same fish in cool water stay pale, hover rather than roam, and pick at food instead of surging for it as a group.
Heating a rasbora tank properly
Target 73 to 79 degrees Fahrenheit and set the heater while the tank cycles, confirming the reading on a thermometer at the far end from the heater. Harlequins are small, active fish usually kept in groups of eight or more, and the whole school shares one body of water — so stability matters as much as the number itself. Place the heater where filter flow washes past it, avoid a spot the fish can wedge behind, and recheck the temperature after any water change, since a large top-up of cooler water can dip a modest tank noticeably. If the school will live with tankmates, choose companions whose warmth needs overlap the harlequins' range rather than bending the temperature toward a species that wants it cooler.
Signs the temperature is wrong for the school
Cold harlequins drift apart. The shoal loosens, the black wedges fade to grey, and the fish hang at midwater with little of their usual zip, often refusing the morning feed entirely. Overheated water produces frantic surface-skimming and rapid breathing instead. In either case, check the thermometer before touching anything else, adjust the heater gradually, and give the school a day at the corrected temperature to recover its rhythm. If the paleness and lethargy persist in properly warm water, or individual fish waste away while the rest eat, that is the moment to ask a veterinarian who sees fish or your local aquatics shop what else could be going on. For the next setup question that follows, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.