Fish You Can Keep Without a Heater

Written 2026-11-10

White cloud minnows and medaka can live happily with no heater at all, running on room temperature alone. In the Water Kiwoogi simulator, a 45cm tank with no heater at 22°C room temperature runs for 90 days with six white clouds and six medaka, and neither group ever gets a single stress warning. Put a single betta in the same tank instead, and it dies in just two days — 24~30°C is its comfort range, and 22°C is already 2°C below that.

Why some fish don’t need a heater

In this simulator every fish has a comfort range stored as [minimum, ideal, maximum] degrees, and temperature stress is zero as long as the water stays inside that range. White clouds tolerate 14~24°C, and medaka go even further, down to 4°C and up to 32°C — room temperature at 22°C sits comfortably inside both ranges. A betta’s range is 24~30°C, so 22°C falls 2°C below its minimum. Every hour spent below that minimum chips away at health, and once the damage outpaces recovery, health keeps dropping until the fish dies.

The numbers

Same 45cm tank, same sponge filter, same 22°C room temperature — only the substrate and the fish species changed across a 90-day run. White clouds and medaka went in on day 15 and stayed healthy to the end; the betta went in on day 15 too.

Cycling also finished normally with no heater — ammonia and nitrite reached zero on a test kit and stayed there from day 23 for the white cloud tank and day 21 for the medaka tank. Peak nitrite stayed low too, 0.76ppm and 0.2ppm on a test-kit scale, well within safe limits for both. The betta’s death wasn’t a water quality problem at all — temperature was the only cause logged.

Picking a species for a heaterless tank

If you’re planning a tank with no heater, the key is choosing a species whose natural range already sits near room temperature. White clouds and medaka both have a minimum comfort temperature below typical indoor room temperature, so they’re safe choices. Fish commonly sold as “tropical” — guppies, neon tetras, bettas, gouramis and the like — usually have a minimum around 22~24°C, which can be risky in an unheated room. If you already have a tropical fish in a heaterless tank, or it looks sluggish, it’s worth talking to a fish store or an aquatic vet about adding a heater or moving it somewhere warmer.

The button below runs this article’s 45cm tank · no heater · 22°C room · 6 white cloud minnows setup. Swap in a betta and you can watch how fast it struggles under the exact same conditions.

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