How Long Will Tropical Fish Survive Without a Heater?

Tropical fish do not swim freely throughout the ocean; instead, they stay in one particular habitat where they’ve adapted to the surrounding temperature. When water temperatures fall below the critical level for a tropical fish, it can lead to hypothermia, cold shock, stress, or death — but just how long can a tropical fish survive without a heater?

Tropical fish can survive for up to two days without a heater. However, low water temperatures increase stress levels in tropical fish, making them more susceptible to illness. Different species have different suggested temperatures, but generally, you should keep the water between 75°F and 80°F.

In this article, I’ll explain why tropical fish need heaters, how low temperatures affect them, and the expected life span for tropical fish suffering from hypothermia or cold shock. Finally, I’ll offer tips for keeping your fish safe when electricity isn’t available (such as during a power outage). Read on to learn more.

Why Tropical Fish Need Heaters

Fish are ectothermic creatures, meaning they depend on the water temperature to maintain their body temperature.

Tropical fish survive in environments between 75°F and 80°F (23.8°C to 26.6°C), with most ideal water temperatures around 78°F (25.5) depending on the species. Most people don’t keep their living environments this warm, and temperatures fluctuate throughout the day and night. As such, it’s vital to use heaters to ensure the temperature remains consistent.

All fish utilize countercurrent exchange to pick up more oxygen, but tropical fish also use this design to maintain their body temperature. Their blood flows in the opposite direction of water flow against their gills, allowing for efficient heat transfer.

The importance of stable water temperatures to tropical fish means that it’s one of the most essential things a fish owner should maintain to keep their pets healthy. Keep in mind, too, that one heater may not be enough to heat larger tropical aquariums, so you may need to invest in two or more heaters.

Tropical Fish Can Tolerate Minor Environmental Changes

Minor fluctuations in a tropical fish’s environment aren’t too much of a concern regarding survival. Generally, these warm-water fish can endure temperatures between 10 and 15 degrees below their recommended temperature. However, while they’re unlikely to die, it could affect their overall health by increasing stress and reducing their lifespan if exposed to low temperatures for long periods.

Some tropical fish can withstand temperatures as low as 55°F (12.7°C) at least for a few hours. With that said, the more the temperature drops below the recommended minimum, the harder your tropical fish’s body works to keep its organs functioning. Therefore, if you can’t reheat your tank within an hour or two, you need to seek alternative ways to get the aquarium back up to temperature as soon as possible to avoid stressing the fish.

I’ll explain how to maintain water temperatures without a heater later in this article.

Low Water Temperatures Stress Fish

When water temperatures fall below a critical level for a particular tropical fish species, they can die. Even if they survive the cold shock, fish become stressed, making them more susceptible to illnesses such as bacterial and fungal infections.

Fish suffering from cold shock or hypothermia may show the following symptoms:

  • Slow Moving or Lethargic
  • Depressed
  • Refusal to Eat

Hypothermia can also cause issues with a fish’s metabolism and lead to immune system dysfunction.

Signs of Stress in Fish

A paper published in Acta Amzonica, a peer-reviewed scientific journal, showed that Amazonian fish species, matrinxã, did not show a physical stress response to cold shock. However, plasma cortisol and glucose levels increased as the fish made their recoveries. This indicates that even if fish don’t appear distressed at first, stress levels may increase as they recover.

Signs of stress in fish include:

  • Frantic Swimming
  • Running Into Tank Floor or Walls
  • Rubbing Against Substrate or Rocks
  • Pulling Fins Inward

Although a small amount of stress typically isn’t fatal, the more a fish experiences stress, the less likely it is to cope with it later. Therefore, you should remove sources of stress as soon as possible. If you suspect stress is caused by the water temperature, bring the tank up to the suggested temperature.

However, keep in mind that eliminating stress doesn’t guarantee healthy fish. Reducing stress simply increases the chances that your tropical fish stay healthy, the same way that reducing stress can increase lifespan in humans.

Tropical Fish Life Span with Hypothermia

The mortality rate of tropical fish exposed to cold waters depends on the species, water temperature, and duration of exposure.

Here are two examples from peer-reviewed journals:

  • Yellow Croakers – A peer-reviewed journal, “Foods,” published by the Multidisciplinary Digital Publishing Institute, concluded that yellow croakers survived for 36 hours under the highest critical temperature of 46.4°F (8°C) when dropping the temperature in increments of 3°C per hour. However, the fish did experience stress, though it was mitigated once moved to a warmer area, around 59°F (15°C) for recovery.
  • Zebrafish – The Journal of the American Association for Laboratory Animal Science conducted a study in which zebrafish survived exposure to hypothermal shock for up to 12 hours. Most fish recovered after 60 minutes of exposure. However, there was a correlation between the age of fish and survival rates, with younger zebrafish surviving for up to 12 hours and older fish expiring in as little as 20 minutes.

It’s important to note that these studies did not monitor the surviving fish afterward. Many fish end up with Ichthyophthirius multifiliis, a parasitic infection that leads to tiny white spots on the body. Stress is one of the primary factors leading to an ich infection in fish.”

Ich infections may not appear for up to two weeks after a stressful event. If left untreated, ich can lead to 100 percent mortality in an aquarium. Monitor your fish closely for any signs of illness for two weeks if they’re ever exposed to temperatures lower than recommended.

Heating a Tropical Aquarium During a Power Outage

Sometimes situations occur that are beyond our control, such as power outages. It’s important to prepare yourself to avoid losing or harming your fish in such an event.

If the power goes out, any electric equipment, including your aquarium heater, pump, and filter, cannot operate. As a result, the temperature and oxygen levels in the tank begin dropping rapidly.

Here’s what you should keep near your tropical aquarium in case of an emergency like this:

  • Thick, insulated blanket
  • Plastic bags
  • Thermometer
  • Battery-operated pump, filter, and heater
  • Batteries

While most tropical fish can withstand temperatures in the low 60s and high 50s (Fahrenheit) for several hours, it’s best to keep temps as close as possible to their suggested conditions.

Here’s how to keep the aquarium at an adequate temperature during an emergency:

  1. Insulate your aquarium with a blanket to minimize heat loss. Be sure to unplug the lights (to avoid creating a fire hazard once power is restored).
  2. Heat water on a gas range, wood-burning stove, or fireplace and carefully pour it into plastic bags. Avoid overheating it, or else you could harm your fish. Use a thermometer to monitor the temperature.
  3. Use a battery-operated pump, filter, and heater, if possible. It’s best to have these options available to maintain proper oxygen, hygiene, and temperature.
  4. Maintain your filter system by removing the media. Rinse it in aquarium water, then place it in the tank to preserve beneficial bacteria. You can put it back in the filter system once the power returns.

Once you’ve exhausted all options, the only thing to do is wait patiently and keep the tank insulated. During this time, avoid further stressing your fish by not disturbing them. You can even avoid feeding them for up to a week, if necessary.


Tropical aquarium fish may seem like challenging pets, but the truth is that they’re actually quite resilient. While exposure to low temperatures can lead to stress and illness, some species are capable of surviving for up to 48 hours in temperatures as low as 46.4°F (8°C).

Of course, it’s not recommended to expose your tropical fish to temperatures below or beyond their suggested conditions. However, if you’re ever in a situation where electricity isn’t available, a little planning and preparation can help you avoid subjecting your fish to a harmful environment.


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