What Does a Filter Do for a Fish Tank? [Answered]

Keeping a fish tank is a big responsibility. You’ll have to look after every need of your fish — from their food to their environment. The maintenance of their surroundings is in your control, and you must know all the dos and don’ts related to this.

For instance, you may be thinking about if you really need a filter for your fish tank. Why don’t you just take the fish out and change the water? That’s simply not how you care for fish and today we’ll learn more about this.

And the main focus will be, as the title suggests, finding out what role does a filter have in a fish tank.

Why Do I Need a Filter for My Fish Tank?

A filter keeps the aquarium water clean, removing debris such as fish food particles and fish waste. It also aerates the water, helping the fish breathe as they need Oxygen too. Finally, it helps keep nitrates, nitrites and ammonia levels under control.

Fundamentally, fish tank filters do the same for fish as what trees do for humans — provide clean air to breathe.

The thing is, your aquarium water is continuously being contaminated by excess food, fish waste, decaying organic matter, debris, and free-floating particles. The filter cleans that up constantly, keeping the water clean.

what does a filter do for a fish tank 1

Have in mind though that the filter itself needs to be cleaned periodically. Both internal and external filters need the media cleaned regularly in order to actually remove all the debris that they’re collecting.

Still not convinced that you need a filter? Check out the following beneficial functions of a fish tank filter in an easier to read, bulleted list:

  • A fish tank filter allows biological filtration to take place in your aquarium
  • It helps in the removal of toxins such as ammonia and nitrates, whose excessive concentration in the water can cause ammonia poisoning
  • It prevents the formation of cloudy aquarium water by removing decaying food, floating particulates, and other harmful organic matter in the water
  • It provides a safe environment where it eliminates the growth of harmful bacteria but at the same time also allows space for the growth of beneficial bacteria
  • By creating surface agitation, the filter assists in oxygenating the water

Types of Filtration Method for Your Fish Tank

Even though the simple answer to whether you need a fish tank filter is yes, the choice also depends on the type of aquarium you have.

Using a fish tank filter is the easiest way to provide biological filtration for the aquarium. The following are the ways these filters help clean the water.

Biological Filtration

As already discussed, no matter what type of aquarium you have, some sort of biological filtration system is necessary.

Defined in the simplest of words, biological filtration helps reduce toxins in the water. So, what’s the science behind it?

Actually, your fish constantly excrete waste into the water. This, combined with debris, decaying organic matter, and excess food makes the water so toxic that it can even kill the fish.

This is where biological filtration comes to the rescue as it coverts the toxins into nitrate, and then it becomes harmless.

However, you cannot let this nitrate be in the water for too long as it can also cause harm to your fish. Nitrate is used by plants, for example, as food. This is why having a heavily planted tank will make things easier for any aquarist – plants help filter the water too!

Mechanical Filtration

aquariums with filters and light

Unlike biological filtration, mechanical filtration is not “necessary” for all aquariums, but it is the best way of maintaining water clarity.

A mechanical filter works by pushing the water through a filter media that acts as a strainer to catch free-floating particulates.

You can also call this process particulate filtration. This type of filtration will trap the excess food particles and debris before they decompose into ammonia. The quality of mechanical filtration depends on three factors:

The fineness of the filter media

Mechanical filters have several layers of media starting from coarser media and moving up to a finer one for best results

Passage of water

The water must pass through the media in adequate amounts. If it moves too slowly, then it will allow the debris to settle in the water. But if it goes too fast, the filter might not be able to collect all the waste

Cleanliness of the filter

Fish tank filters also need maintenance because too dirty, choked or damaged filter media won’t be able to perform its function efficiently.

Maintenance becomes essential especially in the case of internal filters since the filtering elements are still inside the aquarium and can build up to harmful numbers if the filter itself is not properly cleaned on a regular basis (between once a week to once a month, depending on the amount of fish you have).

Chemical Filtration

This type of filtration will solve all the “chemical” problems present in the aquarium water. In other words, chemical filtration can remove heavy metals, ammonia, specific toxins, and other dissolved organics through processes like ion-exchange resins.

These filters are provided with activated carbon or chemical resins that extract toxins from the water until they become saturated.

Chemical filtration will also remove any medication after it has served its purpose and will also purify the source water before it goes into the tank.

As a safer alternative to chemical filtration (but not as complete), you can opt for UV sterilization in your tank. This can be used long term, while chemical filtration is used only on a if needed basis and it’s best to avoid it with a healthy aquarium.


A filter is extremely important in keeping your water clean and your fish healthy and sound by removing excess debris and harmful substances and aerating the water.

While advanced aquarists who pay close attention to the water parameters can go without a filter if they have a heavily planted tank and an underpopulated aquarium, it’s easier to just use a filter to help.

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