The short answer for otocinclus
Yes, and the reason is worth understanding, because it shapes how you run the whole tank. Otocinclus — the tiny algae-grazing catfish sold as otos — need warm water, roughly 73 to 79 degrees Fahrenheit, but what they truly cannot abide is change. Otos are delicate fish with a well-earned reputation for thriving in settled tanks and quietly dying in unsettled ones, and an unheated aquarium delivers exactly what they handle worst: a slow temperature slide every night and a climb every afternoon, forever. The heater's real job in an oto tank is not warmth alone; it is a flat line.
Why otos are built for steadiness
Wild otocinclus live in slow, vegetation-choked streams and flooded margins across South America, in water that is warm, soft, and remarkably constant from day to day. They spend their lives rasping algae and biofilm from plant leaves, and their small bodies and simple digestion are tuned to that unchanging world. Almost everything about keeping them follows from this: they want a mature tank with established grazing, a group of their own kind around them, water that does not swing in chemistry — and water that does not swing in temperature either. A fish that experiences its environment as one long stable afternoon at home will experience your living room's daily thermal roller coaster as a permanent low-grade emergency.
Heating with an oto's nerves in mind
Set the heater for the middle of their range — around the mid-70s — rather than the edge, so the tank has room to absorb a warm day without overshooting. Install it well before the fish arrive and log the reading morning and night for several days; you are not checking a number, you are proving a pattern. Position the heater in the filter flow so warmed water spreads evenly, because otos rest on leaves all over the tank and should not have to choose between a hot corner and a cold one. During water changes, match the new water's temperature before it goes in — pouring in a bucket several degrees cooler is precisely the kind of shock this species absorbs badly. If the tank also holds hardier fish, set the temperature for the otos; they are the residents with the least capacity to adapt.
Reading otos that are struggling
Healthy otos are busy and round-bellied, working a leaf together, their color a clear pattern of brown and cream. Otos in swinging or cold water go still and pale, cling to one spot, and their bellies pinch inward as grazing stops. Because they decline quietly, check on the group deliberately rather than waiting for drama. Correct temperature problems slowly, and if the group does not resume grazing within a day or two of stable, correct warmth — or fish keep dying one by one while the water tests fine — that is the point to consult a veterinarian who sees fish or an experienced aquatics shop, bringing your temperature log with you. Keep writing those readings down, and put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.