How Long Can Fish Go Without Oxygen? Survive A Power Outage!

When I look back, I realize my life has changed quite a bit after I started keeping fish at my place.

For example, usually, when the light went out, my typical reaction was:

“Oh no! I was just about to beat the boss (Obviously, in a game).”

But, now, if the power goes out, my heart skips a beat, and I am in panic mode.


Because without a proper functioning aquarium, the fish are in danger.

The first time this happened, the first question that popped into my mind was:

“How long can fish survive without power?”

Google will tell you that your fish can survive without power for about 2 days. However, the truth is that this figure can vary a lot based on various conditions such as the quality of water, the number of fish, the size of the tank, etc.

So, what actually happens when the power goes out? How can we keep our fish safe? What precautions can we take?

Thankfully, after a great deal of research and personal experience, not only I have an answer for you, but also some precautions and solutions to take care of your fish in case of a power outage.

How to Survive a power outage- Aquarium (Video)

But, first, we need to understand the purpose of an aquarium.

What is the purpose of an aquarium?

There are two main functions of an aquarium as follows:

  • Pump water or air
  • Regulate Temperature

An aquarium pumps water so that the water reaches an air-water interface and becomes saturated with oxygen. This water, now rich in dissolved oxygen, is circulated throughout the aquarium so that the fish can breathe.

The secondary function of the aquarium pumps is the filtration of the water to keep it clean for the fish to live in.

Apart from pumping water, the aquarium also regulates the water temperature. This is essential because we all know that fish are cold-blooded animals and require a certain level of water temperature to survive.

What happens to an aquarium during a power outage?

During a power cut, all the functions of the aquarium stop.

Meaning, the water does not reach the air-water interface and is not circulated.

Therefore, the fish have to survive with the amount of oxygen left in the water. And how long your fish can survive depends upon the type of fish, the number of fish, and the condition of the water.

Moreover, the temperature of the water would start to rise or fall depending upon the ambient temperature. And if the surrounding weather is extreme, your fish will have a hard time surviving.

Needless to say, without filtration, the water will start to get dirtier, making it harder to live in for the fish.

How do I keep my fish alive during a power outage?

Now that we know the immediate dangers of a power outage, it is time for us to educate ourselves about everything we can do to keep our fish survive without electricity.

Lower the water level

We have already learned that without electricity, the oxygen in the water starts depleting because of being consumed by fish and organic waste.

When this happens, the oxygen at the bottom of the tank becomes extremely low. Only the top few inches of the water have some oxygen left.

There are certain species of fish like the Cory Catfish or the Bristlenose Catfish, swim to the top surface, and start gulping air. Such species can handle such oxygen-deficient environments well and survive for a long time.

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However, not all fish can do that.

To help those fish, you will need to lower the water surface by removing the water from the tank until there is only enough water left so that the fish are able to swim upright.

So, when the fish move, they will agitate the water; thus, increasing the oxygen levels.

Remove filtration

Aquariums have filtration bio wheels to remove waste and unwanted toxins from the water. However, without any power, these very filters can pose danger to the fish.

Remove the filtration bio-wheel from the aquarium so that the waste collected in them does not increase the toxicity of the water.

On the other hand, if you are using canisters for filtration purposes, disconnect them.

Use a siphon tube

If you suspect that it will take longer to get the power back on you will need to do the following to make sure the fish are getting enough oxygen:

Catch all the fish, one by one, from the aquarium and put them in an empty water tank. Once you’ve placed all the fish in the empty tank, use a siphon tube to spray the water from the water tank into the empty tank (now containing the fish).

Fill the tank enough so that the fish are able to swim upright.

By spraying the water into the tank, you are increasing the air-water exchange that will increase the oxygen content of the water. The higher you can spray the water from, the better.

You can also use freshwater to fill the tank. But, it is better to use water from the aquarium because the fish are accustomed to that.

Moreover, make sure the tank is large enough to keep the fish depending upon their size.

Note: It is important to cover the tank with a net so that the fish do not jump out.

Do not feed the fish

Never feed your fish when the power is out, as it can make things much worse for them.

If you feed them, they will eat the food, and their metabolism will increase, causing them to consume more oxygen, the one commodity that water is lacking.

Another problem the food poses is that of contamination.

The leftover food, along with the fish waste, will contaminate the water. Moreover, leftover food and waste materials are organic matter, meaning they will consume what’s left of the oxygen in the water.

You shouldn’t be concerned about your fish starving to death because fish are known to survive weeks without any food.

Regulate Temperature

If you are living in an environment having a moderate temperature, then you do not need to worry as the water temperature will remain more or less the same.

However, if you live in places with extreme weather conditions, be it too hot or cold, you will have to take certain steps to ensure that the water temperature is adequate for your fish to live in.


If you are living in a colder environment, you can take a bowl of water and boil it. Pour the water into the water tank containing the fish so that the hot water can increase the temperature of the cold water so that the fish can easily survive in it.

But make sure you know the initial temperature of the water and slowly add the hot water, so that you don’t overdo it.


If the climate you are living in is hotter than normal, you need to lower the temperature of the water.

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Pour some ice cubes in a plastic bag or a plastic bottle and place it in the water tank with the fish. The ice cubes will lower the temperature of the water.

Again, it is important to ensure that you do not overdo it. The temperature of the water needs to be moderate for the fish to survive.

Other steps

The steps discussed before are in case the power outage is a sudden one and will come back after a few hours or maybe after a day.

However, if the power outages are frequent in your region, or you are suspecting a power outage because of a thunderstorm, here are certain things you can do to keep your fish secure:

Generator/back-up battery

You can get a generator for the aquarium in case of emergencies. However, running a generator has its own problems.

Running a generator can be expensive, especially if you are running a high-end aquarium with a lot of features.

To avoid losing a lot of money, you can run the generator for an hour or so and turn it off for two hours. Repeat this cycle until the power is back on.

This will maintain the oxygen levels and temperature to a moderate level ensuring your fish will survive.

You can also get an inverter with a couple of batteries so that the aquarium pumps and temperature regulators keep on working. These batteries can power your aquarium for up to 12 hours, depending upon the batteries and the aquarium.

However, you should keep regular checks on the generator or the batteries to ensure that they are working. Otherwise, during the time of need, the generator might not work because it is out of gas, or the batteries might be dead.


How long can fish live without a filter?

Without filtration, the good bacteria in the aquarium start to die within 30 minutes. And without good bacteria, the nitrate levels in the water, start to rise.

High ammonia and nitrate levels will cause the water to become toxic and harmful for the fish.

To keep the fish alive, you need to make sure the nitrate level of the aquarium water is close to zero.

You will need to monitor the nitrate level of the water and if it starts to increase, do a 50% water change that will lower the nitrate level.

Keep doing this until the filtration is back on.

Can I use hydrogen-peroxide to add oxygen to the aquarium in case of a power outage?

Hydrogen peroxide (H2O2) can be used to add oxygen to your aquarium.

You should add one teaspoon of Hydrogen peroxide (3%) for every ten gallons of water. Take out a gallon or so of water from the tank and add Hydrogen peroxide, mix it, and then add the solution into the tank.

Note: For 6% Hydrogen peroxide, add half a teaspoon per ten gallons, for 9% Hydrogen peroxide, one-third, and so on.

This will provide your fish with enough oxygen for up to 12 hours.

However, this method is not recommended because it has its own risks. If overdone, it can harm your fish.

Therefore, only do it if you have no other option.

It would be better if you add a lesser than the required amount first and see if the fish are okay and are not facing any issues. Once you are sure that the fish are fine, you can go on to add the recommended amount of Hydrogen peroxide.

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Nadine Oraby

My name is Nadine; I am a passionate writer and a pet lover. People usually call me by the nickname “Joy” because they think that I am a positive and joyful person who is a child at heart. My love for animals triggered me to create this blog. Articles are written by vets, pet experts, and me. Thanks for visiting. Your friend, Nadine!

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