How Long Can a Fish Tank Filter Really Be Off?

Your fish tank filter can be off due to various reasons, and your primary concern will then be how long your fish tank filter can be turned off without killing your fish or other livestock.

A fish tank filter can be left off for up to 12 hours without harming your fish. After twelve hours, some of the filter’s nitrifying bacteria may begin to die, but they will quickly regenerate once the filter resumes operation. Adding a battery air pump and a sponge filter or airstone to a fish tank can provide oxygen to fish during a power outage.

Beneficial Bacteria

The function of a fish tank filter is to provide mechanical and biological filtration for your fish tank. The mechanical filtration will remove particles from the water and basically clean the water. Fish produce waste in the water, and the most important aspect of a fish tank filter is establishing bacterial colonies that remove this waste. It regulates the nitrogen cycle, in which ammonia is converted to nitrite and then to nitrate.

These bacterial colonies in the filter media of your fish tank filter require oxygen to survive. When the fish tank filter is turned off, there is no longer any water flow to provide oxygen, and they begin to die off.

The rate at which the bacteria dies off is actually not established, but it is fairly slow, and they will quickly regenerate once the filter is turned back on. You should keep the filter media wet to prolong the life of the bacteria, which should not be a problem in most types of filters. You should keep this in mind if you are moving fish tanks – keeping filter media wet will keep most of the bacteria alive.

Oxygenation

Another one of a fish tank filter’s primary functions is to circulate water and agitate the water’s surface in a fish tank. This increases the oxygen level in the water, which is critical for the survival of your fish. The majority of aquarium fish that die due to an off fish tank filter do so due to a lack of oxygen in the water.

How long your fish can survive with an off fish tank filter is dependant on how heavily your fish tank is stocked and the size of your fish. Small fish rarely show any signs of oxygen deprivation, while bigger fish can quickly degenerate.

If you have a planted tank full of tetras, they can survive an off fish tank filter for days without any issue. If you have an overstocked Lake Malawi setup full of peacocks or mbunas, you can have an issue with fish dying from oxygen deprivation within hours when your fish tank filter is off.

A real lifesaver is a battery-driven air pump with a sponge filter or an airstone in this type of situation. The sponge filter or airstone will provide much-needed oxygen to the tank, and this will keep your fish alive.

Some air pumps have built-in battery backup, seamlessly switching from mains power to battery power when the power goes out. This can keep a sponge filter running in a power outage, providing filtration as well as oxygenation.

Can I Turn My Fish Tank FIlter Off at Night?

There can be various reasons why you want to turn your fish tank filter off at night. The fish tank might be in a bedroom, and the hum of the air pump or filter can be disturbing.

You can turn your fish tank filter off at night if you have a light to moderately stocked tank with small fish that will not show any signs of oxygen deprivation during the night. 8 to 12 hours with an off fish tank filter, in this case, will not do any harm.

Do not, however, turn off your fish tank filter at night if:

• Your fish tank is heavily stocked with fish.
• You have fish 5 inches or bigger.
• You have a marine tank.

Do I need to leave my fish tank filter on all the time?

You need to leave your fish tank filter on all the time if you have a heavily stocked fish tank or a tank with big fish, which is dependant on the oxygenation provided by the fish tank filter. However, if you have a planted tank with smaller fish species, your fish tank filter does not need to be on all the time.