How Long Will Your Fish Survive Without Oxygen and Filter?

This question is obvious for an ornamental fish keeper. If there is a power blackout, you will be panicking, "what happens to my fish"? Like most living creatures, oxygen is necessary for your fish. Aquarium fish require dissolved oxygen(DO), as they can absorb it directly into their system.

An air pump supplements the DO in your tank. The unit circulates water and breaks its surface tension increasing oxygen absorption. Another source of oxygen in the tank is live water plants, as they produce oxygen as a by-product during photosynthesis.

fishes in aquarium 1 Source:

So, depending on the aquarium system, your fish can survive more days without oxygen. However, a fish can only stay for two days without oxygen in a typical tank. Also, a fish can survive 3-7 days without a filter, but this depends on the water quality in the tank.

How to Know their Low Oxygen Levels

It's not automatic to realize there are low oxygen levels unless you take measurements regularly or observe fish behavior. The best DO levels in your aquarium should be between 5-and 6 ppm. When oxygen levels lower, your fish will start swimming less active and eat less. Other fish behaviors include;

  • Labored breathing
  • Rapid gill movement
  • Gasping at the water surface
  • Fish resting for more extended periods

Emergency Steps

If you notice your fish having such behaviors as gasping at the surface, you should take some action. These steps include;

  • Change water to around 50% to reduce the amount containing waste substances as it reduces the level of dissolved oxygen. Freshwater will add more oxygen to the tank as it mixes with the remaining water and also it's capacity to hold more dissolved oxygen especially if it's cold.
  • Lower the water temperatures accordingly by switching off the heater. Low water temperatures will have more dissolved oxygen.
  • Avoid overfeeding your fish, and remove any leftovers in the water.
  • Have an efficient filtration system, that is one that will fully eliminate waste materials in water, and organic matter and promote the dissolution of oxygen.

What Causes Low Oxygen Levels in Water

Several factors deplete oxygen in an aquarium.

  • Overcrowding- keeping a large number of fish in a small aquarium or beyond the recommended stocking density reduces oxygen levels. Each fish has its oxygen requirements, so they will use up all the DO if you keep too many fish.
  • High Temperatures- as the water temperature rises, oxygen solubility reduces.
  • Lack of Water Movement- water circulation leads to high oxygen levels in the water. If water remains stagnant, then the oxygen levels will be low.
  • Waste Accumulation- the breakdown of waste materials by beneficial bacteria requires oxygen. So, if waste substances accumulate, they use up more oxygen and lower the levels.
  • Presence of Live Plants- plants produce oxygen as a by-product during the day but later consume it at night. Even when light levels lower, plants will also use up oxygen, including algae.
  • Presence of Some Chemicals – there are chemicals that, like additives, affect the oxygen-carrying capacity of water.

How Long Can Fish Live Without a Filter?

A water filter is necessary for your tank. Depending on your fish needs and water quality parameters, fish can stay for one week without a filter. Though this doesn't mean it’s healthy to leave your fish without a filter.

First, the bacteria will die off in 30 minutes after eliminating a filter( which supplies oxygen). The fish's survival will first depend on the water depth. In a shallow tank, the fish will cause water circulation as they move and increase oxygen levels, thus staying longer.

What Happens When Fish Stay Without a Filter

A filter helps break down the toxic compounds into safe substances in an aquarium through a biological system. The filter also eliminates dead decaying organic matter and waste products from the water. So, when you switch off the filter, the following happens;

  • Oxygen Depletion

A filter enhances the oxygen supply in the tank for all creatures. So, removing a filter for a long time leads to oxygen depletion, and fish and other organisms may die.

  • Algal Bloom

A tank without a filter has a lot of waste buildup and organic matter. A large amount of nutrients causes excess algae growth.

  • Excess Toxins

Without a filter, ammonia and nitrate levels will accumulate to high levels. These excess toxin levels will kill your fish and lead to losses.

  • Loss of Healthy Bacteria

A filter plays a significant role in maintaining a healthy aquarium environment through beneficial bacteria. When you turn off the filter, many toxins accumulate, killing the beneficial bacteria.


Several factors affect your fish's survival without oxygen or filter. Some of the elements are the type of fish species, the tank size, and population. Some species survive longer in these conditions than others. However, it doesn't mean you should leave your fish without oxygen and a filter. Ensure you care for your creatures by providing the necessary water quality parameters and a favorable environment.

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