How Long Can Fish Survive in a Tank Without Filter?

The oxygen level in a fish tank is crucial for the fish’s survival, and every aquarium owner wants their fish to live and thrive in a conducive environment. However, there are certain situations where the filter malfunctions or there is a power outage, so understanding what happens to fish when there is limited oxygen supply is important.

So, how long can fish survive in a tank without a filter? Generally, when the fish are in such an artificial setup, they can survive for up to 7 days without a filter if there is adequate oxygen available in the water. Chances are that they may die within 7 days, if the oxygen levels in the water are not sufficient for them to breathe or other chemicals build up in the water, to toxic levels.

However, there are so many factors to consider before determining how well the fish can survive in such an environment. We need to consider the type of fish, tank’s size, the present biomass, and other factors.

If you are worried about your fish’s survival in a tank without a filter, this article is the right guide for you. We investigate how important the filter is and what happens if it breaks down or during a power outage. Read on as we expound on these and more concerns in the coming chapters.

Is the Filter That Important?

A filter is a perfect way to create a natural environment for your fish since it resembles the typical aqua life that they are used to. When you keep the fish in an aquarium, you have to ensure enough oxygen and food supply. The filter comes in handy to create a cycle for them to survive in. So, how does it work?

The fish release ammonia, a toxic substance that needs expulsion from the tank. Naturally, these toxins are carried away by the water bodies’ currents because the fish live in rivers or oceans. Therefore, when you hurdle them in one place, you need to create an artificial detox means.

The filter’s main role is to sieve out any contaminants that may hinder your fish’s survival. There are mechanical, biological, and chemical filters, all of which effectively provide the required oxygen and decontaminate the tank water. They ensure that the aquarium is always clean and that all the toxins are expelled.

Most importantly, filtering aerates the tanks by providing the oxygen needed by the plants. By constantly supplying it, the fish will breathe better; otherwise, they may slowly suffocate and eventually die. The filter also provides regular water flow, which the fish are accustomed to in their natural setting. Thus, by creating an environment that mimics the one in their ecological system, they will thrive in the tank.

How Long Can Fish Survive in a Tank Without a Filter?

Generally, it is quite dangerous to let the fish go without a tank filter unless you are willing to clean up the tank and change the water frequently. However, whether your fish will survive or not depends on so many factors. Such factors include the biomass (how many fish you have), the filter’s role, and the type of fish you have.

First, not having a filter is unhealthy for your fish, especially if your tank is small but is full of fish; they will compete for the available oxygen, which may be fatal. However, if you have a few of them in a large tank, their survival chances increase.

Secondly, you need to consider your fish species. While some can bear low levels of oxygen, some may not survive for long. When the water is still, the fish that cannot swim to the surface will struggle to survive. Another factor to consider is the water’s temperature, given that when the water is cold, it retains more oxygen than warm water.

Immediately the filter fails, the bacteria will suffer first; if these bacteria are necessary for the fish, it will affect them too. Moreover, when there is limited oxygen, the fish will survive for some time as they try to utilize the remaining oxygen available.

When this also goes down, they strive to get to the surface, hoping to find more air. This process is tedious and stressful for the fish. In most cases, they can struggle for just a few days until they start suffocating.

Depending on the factors, it is best to ensure that your fish do not go for more than two days without a filter. This may be the maximum number of days they can stay alive.

Will My Fish Be Okay Without a Filter For a Day?

It is common for there to be filter malfunctions or power outages; so, there is no need for alarm when the filter stops for a few hours or a day. Given that the filter has been running, some amount of oxygen is available in the tank.

If it suddenly stops, the fish will still have some oxygen to use. If not, they will try to come up to the surface for more air. However, note that it is best to try and solve the situation as soon as possible; otherwise, you may be putting your fish at risk.

Will Fish Die If the Filter Stops Working?

One argument among many people is that if fish can survive in shallow ponds even without filters, they may still survive without one in the tanks. But note that in such environments, there are so many factors that promote their survival.

Besides, when the water level is low, there is increased air circulation to survive. As we have noted, the fish are likely to die from suffocation if the filter does not work for some days. Therefore, it would help to start considering how to salvage the situation before it’s too late since you may only have a few days to keep them alive.

For their survival, there are remedies you can put in place to alleviate the situation. For instance, if there is a power outage, you need to act fast to save them. One remedy is the generator or an inverter; this way, you can still filter and heat your water.

The inverter is cheaper than the generator, but both will serve you until the power comes back. When you have either of these machines, you will always be ready in case of such emergencies.

Not all aquarium owners can purchase the generator or inverter; hence, you can deploy manual water filtration. In other cases, your filter may suddenly break down. The way to go is to invest in an extra tank for your fish, and you only need to keep it half-full and use it to balance the biomass.

By reducing the fish population, you will reduce the overcrowding and struggling for air. The fish will swim freely, and it will buy you some time before the filter works again.

Similarly, it would help to avoid placing live aquatic plants when the filter isn’t working; they will compete with the fish for the little oxygen available. In case these processes are too much for you, you can always find an alternative; consider asking a friend who has a backup system.

The goal is to act fast, bearing in mind that the fish can only survive for a few days before they suffocate and die.


We have noted that having a great filter is crucial for your fish; it will help eliminate wastes from your tank and create a suitable environment for them. Otherwise, you will lower the tank’s water quality; therefore, you always need to ensure that your filter works properly.

However, there are some unavoidable circumstances where the filter will suddenly stop working. There may be sudden power outages or filter malfunctions. But there is no need to panic. You can still keep your fish alive for a couple of days, but you need to act fast to solve the issue; otherwise, you may lose your fish.

Bal Kang

Bal Kang is an owner of several pets including reptiles, cats and dogs. An avid writer, who loves to share her insights into caring for pets.

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