What Does an Aquarium Filter Do in a Fish Tank?

If you are just getting into the world of fish and aquariums, you might wonder whether or not your fish tank needs a filter.

Some sources will tell you that particular fish and aquarium sizes don’t need filters. Anybody who tells you this is wrong.

Aquarium filters serve different vital purposes, and it’s all about water maintenance and fish health.

Let’s find out and answer the question ”what does an aquarium filter do in a fish tank”.

There are 6 main functions that an aquarium filter serves, with one other purpose—depending on what kind of filter you get.

1. Removal of Solid Debris

The function of a water filter is to remove the solid debris. This is often referred to as mechanical filtration.

Most filters come with porous sponges or mesh that remove fish waste, uneaten food, and dead plant matter from the water.

This is very important because fish waste, decaying food, and plants release toxic compounds into the water. Therefore, these need to be removed before they start to break down. This also helps prevent the water from becoming cloudy.

2. Nitrogen Cycling

The next purpose that a filter serves in a fish tank concerns the nitrogen cycle. When food, plants, and fish waste break down, they release ammonia. No amount of mechanical filtration will stop this from happening 100%.

A good filter needs to feature biological filtration, which is when beneficial bacteria are present. These bacteria break down that ammonia into less harmful compounds and eventually into totally neutral compounds.

Without sufficient biological filtration, ammonia builds up, and fish die. This is probably the most important job of an aquarium filter—removing ammonia via biological filtration.

3. Removal of Other Compounds

The third type is chemical filtration, which is usually done with activated carbon. This type of filtration is also essential. It helps to remove various compounds in the water, such as dissolved organic materials.

Chemical filtration will remove foul odors and discoloration from the water. It can also remove heavy metals and many other unwanted compounds from aquarium water.

4. Aeration and Oxygenation

The next purpose is to provide the water with sufficient oxygen for your fish to breathe properly.

The right kind of aquarium filter, especially a trickle or waterfall filter, will drive air into the aquarium as they expel water back into the tank. The more air is driven into the aquarium, the better off your fish will be.

To maximize this effect, you may want to connect an air stone to the filter’s outlet tube.

Remember that oxygenation and aeration are two different things; oxygenation refers to the total level of dissolved oxygen in the water, and aeration refers to how well that oxygen is spread out in the water.

5. Water Flow

The fifth and final main benefit of aquarium filters is that they help to produce water flow. Most fish like to have a certain amount of water flow to feel comfortable, whatever they would be used to in their natural habitat.

Moreover, having sufficient water flow is crucial in tanks, mainly to help aerate the tank as discussed above.

Finally, if you have a saltwater tank with corals, they will need decent water flow to drive food towards them.

6. Killing Algae

This final benefit is not one that all filters will provide. If you expect your filter to help kill off algae, you will need one that also includes a UV sterilizer bulb.

UV light sterilizers can kill algae and prevent blooms from occurring.

UV bulbs can also kill bacteria, parasites, and viruses that can harm your fish. Suppose you are looking for a filter with an integrated UV sterilizer.

In that case, you are probably looking for a reasonably large canister filter.


As you can see, aquarium filters serve different crucial roles in the health of your fish and the aquarium as a whole.

Make sure to get an efficient filter that uses in the 3 major types of filtration and one that can handle at least 2 to 3 times the total water volume of the tank every hour. If you do this, your fish and your tank should be healthy.