Keeping Tropical Fish Without a Heater in Winter

Written 2026-10-21

Tropical fish like neon tetras can’t handle a room that drops to 18°C without a heater. We ran a 60-liter tank through the Water Kiwoogi simulator: with a heater holding 25°C regardless of room temperature, six neon tetras stayed perfectly healthy. Without a heater at a room temperature of 18°C, all six were dead within three days, and at 14°C they started dying the same day they went in. Cycling itself took longer too, the colder the room got.

Without a heater, the water just follows the room

In this simulator, tank water settles at the room’s own temperature. A heater warms it toward a target (25°C in this guide), but with no heater there’s nothing to warm it, so the water ends up at whatever temperature the room is. A room with the heat turned down for winter can easily drop below 18°C overnight, and an unheated spare room or a balcony-adjacent room can fall below 14°C — the mirror image of summer heat, except in winter a missing aquarium heater means the water gets colder, not hotter.

What actually happened — wiped out within three days

We set up the same 60-liter standard tank with a sponge filter, ran a fishless cycle with bacteria starter and an ammonia source first, then added six neon tetras (their minimum school size) on day 30, after cycling was done, and watched through day 60. The only things we changed were the room temperature and whether a heater was installed.

With the heater, the water stayed at 25°C no matter the room, squarely inside the neon tetra’s comfortable range (22–27°C), and nothing went wrong over 60 days. Without a heater at a room of 18°C, the water sat 4°C below the species’ minimum (22°C), and the fish started dying the day after they went in — all six were gone within three days. At 14°C, 8°C below the minimum, it was even faster: deaths started the same day they were added, and the tank was wiped out within two days. The colder it was, the faster they died.

Cycling itself slows down too

Before any fish went in, the day cycling finished also shifted — day 15 at a 22°C room, almost the same at day 16 for 18°C, but a clearly later day 26 at 14°C. Filter bacteria grow more slowly in cold water. Starting a new tank without a heater in winter means waiting longer just to reach the point where it’s safe to add fish at all.

So what should you do

The neon tetra in this guide is a typical tropical fish with a comfortable range of 22–27°C — nowhere near tough enough to sit in an unheated room or a chilly balcony all winter. A heater isn’t something you only think about once, when setting up the tank; it’s what keeps the water from drifting away from room temperature every time the weather turns. It’s also worth checking that the heater’s wattage is enough to cover the gap between your room and your target temperature. If you’re looking for a hardier, cold-tolerant species instead, check that species’ own comfortable range in the catalog first, and talk to an aquarium shop or an experienced keeper before relying on it.

The button below runs this guide’s no heater · room 18°C tank — the one wiped out in three days.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.