The short answer for a cherry barb tank
Yes — plan on a heater. Cherry barbs are a tropical schooling fish, and the water they are built for sits noticeably warmer than the air in a typical house. Left unheated, a tank usually hovers in the high 60s to low 70s Fahrenheit, and that is cool enough to dull a cherry barb's color, slow its feeding, and leave it open to the stress-related problems that follow a cold fish around. A modest heater is not an optional extra for this species; it belongs in the basic setup alongside the filter and the lid.
Where cherry barbs come from, and why it matters
Cherry barbs originate in the slow, shaded streams and pools of Sri Lanka, where overhanging plants keep the water dim and the temperature steady in the mid-to-upper 70s. Their whole system — digestion, immune response, the bright red flush a healthy male shows when the school is settled — is tuned to that warmth. Because they are small fish with fast metabolisms, they register a temperature drop quickly, and because they are schoolers, one cold barb usually means the whole group is affected at once: instead of roaming the plants together, they huddle low and quiet.
Setting up heat for the school
Aim for a steady 73 to 79 degrees Fahrenheit. Install the heater while the tank is cycling, before any fish arrive, and let it run for a full day so you can watch the temperature settle rather than guessing on stocking day. Put the thermometer at the opposite end of the tank from the heater; the glass right beside a heater always reads warmer than the corners where the barbs actually swim. Position the heater near the filter outflow so warmed water circulates instead of pooling in one spot, and check the reading morning and evening through the first week — a heater that overshoots is as much a problem as one that underperforms. If the barbs will share the tank with other tropical fish, set the dial for the cherry barbs' range and confirm the tankmates' comfort zone overlaps it.
What cold or overheated barbs look like
A cold cherry barb goes quiet: the males wash out from red toward a dull brown-pink, the school hugs the lower plants, and feeding responses slow to a picky nibble. Water running too warm shows the opposite picture — restless swimming near the surface and rapid gill movement as the oxygen thins. Either picture is a prompt to check the thermometer first, then the heater, before changing anything else, because temperature swings caused by fiddling do more harm than a steady reading slightly off target. If the group stays listless after the temperature is back in range, or fins clamp tight and pale spots appear, that is the point to seek advice from a veterinarian who sees fish or an experienced local aquatics shop. Once the fish are home, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.