What does an Aquarium Filter Do?

If you are beginning your aquarium hobby, then understanding how an aquarium filter works is one of the major milestones.

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The filtration system ensures that the water in the fish tank is healthy for the fish by removing contaminants and purifying it.

In this article, we will clarify several questions you may have about aquarium filters.

What is the Nitrogen Cycle in Aquariums?

You might have heard terms, such as the Nitrification process, New Tank Syndrome, or even Biological cycle being thrown around. Basically, all these terms refer to the same thing: the Nitrogen Cycle.

You are probably asking why is it so important? Well, it is a full-cycle where Nitrogen is transmitted from air to plants in the aquarium to fish to bacteria and back to air. In a natural setting, this system needs no human intervention.

Sadly, in an aquarium, the Nitrification process is less of a cycle and more of a biochemical process that comprises continual degradation of nitrogen from ammonia to nitrite and finally to nitrate.

Fish waste releases ammonia into the water, which can be harmful to the fish. Since this biological toxin is present in an aquarium, the nitrogen cycle needs to work effectively to be able to convert and get rid of these waste products.

Even if it weren’t harmful, it is still not healthy for the fish. Would you want to be surrounded by your poop and pee all day long?

Filtration Methods Explained

Usually, the best aquarium filters handle three types of waste: dissolved waste, solid waste, and biological waste. And this introduces us to the three main types of filtration:

Mechanical Filtration

It is also referred to as physical filtration, and it involves circulating water and straining it through some form of media that functions as a strainer.

The strainer will remove solid particles, such as leftover fish food and fish waste, that cannot pass through the opening of the media.

Mechanical filtration should be the first layer of your whole filtration process, because you first want to remove the floating particles from the water.

The mechanical filtration is composed of filter floss, sponge, or even special filter pads, that will block most of the particles from the water.

A mechanical filter will deliver better results if it is cleaned appropriately and frequently. Regular cleaning of the mechanical filtration will also help in improving water flow through the filter.

The water should pass through the different filter materials in the following order: coarse, medium, fine.

If your filter only has mechanical filtration, you always rinse the sponges in aquarium water and not under tap water. Tap water contains chlorine, which will kill off all beneficial bacteria and it will cause a huge imbalance in the fish tank.

Biological Filtration

This process involves nitrifying bacteria and other microorganisms that will help transforming various highly toxic substances  to less toxic substances. Specifically, it converts the ammonia produced by your fish into nitrite, and toxic nitrite into harmless nitrate.

While Nitrate is relatively harmless, if it is not removed from the aquarium, it can cause liver, kidney, and eye complications for the fish. In some cases, a higher concentration of this substance might prevent the fish grill from absorbing oxygen or even suppress their appetite.

Besides this, the decaying food and other organic matter can cause discoloration in the water.

Biological filtration occurs when the water passes through any surface that encourages the growth of bacteria. Biological filtration is only possible through continuous cycling. That is why your aquarium should have some provision for biological filtration.

If you have a small fish population, the mechanical filtration might be enough. But for efficient waste management, you should combine all two or three filtration methods.

There are many type of natural and artificial biological filter media available on the market. Choose the biological that provides the most surface area for beneficial bacteria colonies.

I use and highly recommend the BioHome biological filter media from Amazon, because with this you can achieve a full cycle – will write about this more below.

Chemical Filtration

Chemical filtration will remove dissolved chemicals from water. Activated carbon or chemical resin will aggressively extract toxins from water until the carbon becomes saturated.

For this reason, you should change your activated filter carbon regularly. If the activated carbon filter becomes depleted, it will leach the toxins back into the water and will cause more harm than good.

It is better to not have a chemical filter at all instead having one that is used up.

Chemical filtration is the last part of the filtration process, and the water should flow through it after the biological filtration. This way, you will be able to extend the lifetime of this filter.

Personally, I don’t recommend using chemical filtration on day-to-day filtration.

First, because it is expensive and need to be replaced often. Second, because it is much effective in removing toxins from the water using an RO/DI system.

You should never use chemical filtration when medicating your tank. Activated carbon will neutralize the medicine, and will have no effect.

Types of Aquarium Filters

There are six common types of filtration systems you can use in your tank. Let’s look at each of them:

Canister Filters: They are powerful filters that are often used in medium- to large-sized aquariums. Canister filters provide good mechanical, biological, and chemical filtration. And since they are placed outside the tank, you can hide them behind or beneath the stand.

They come into action when they are pressurized to channel water through the filter media, rather than through them, making these filters ideal for heavy loads. Their main drawback is that they are hard to maintain.

Sponge Filter: This type of filter is wrapped over a tube from an air pump or powerhead. As water goes through it, the bacteria will flourish, thus, establishing a biological filtration. It can also offer some form of mechanical filtration, only that they clog faster, especially if there is excess debris.

When cleaning this filter, make sure to use aquarium water, as tank water kills beneficial bacteria.

Under Gravel Filter: The UGF is a seasoned aquarium filter that uses an air pump and a plate filter placed under the substrate. The waterfowl generated through the bubbles will suck the water down through the substrate, thus carrying with it particulate matter.

Power Filters: They are also known as hang-on-back filters and are the most popular type of filters we have today. Most aquarium hobbyists love them because they are not only easy to maintain, but they also provide excellent chemical, mechanical, and biological filtration simultaneously.

In most cases, they hang off the back of the tank and pull water up via a siphon tube.

Diatomite Filters: They are specialized aquarium filters that clean the water by getting rid of small particles. These filters are actually similar to diatomaceous earth filters used in swimming pools. Diatomite filters operate by pumping water via a layer of fine particles, which scrub the water clean.

Submersible Filters: Submersibles filters are ideal if you have extra space in your aquarium where water flow can be increased. They also work best in tanks that carry a high bioload and require supplemental filtering.

As the name suggests, they must be fully submerged in the water to work. The good thing with a submersible filter is that it can provide three-phase filtration without the need for manual priming.

What is a Full Cycle?

We are talking about a full cycle, when the filter is able to not only transform ammonia and nitrites into nitrates, but also nitrates into nitrogen and oxygen gas.

These gas will be dissolved in the water and eventually will escape in the atmosphere. The only biological media I know about, that can produce a full cycle is the above mentioned BioHome media.

Achieving a full cycle in your aquarium doesn’t mean that you can skip water changes, but it greatly reduce the stress on the fish, creating a healthier environment and better water quality.

Is a Filter Necessary for Your Fish Tank?

If you are wondering whether or not you should get a filter for your fish tank. The short answer to this is yes. Sure, your aquarium can survive without a filter, but at least you should have some biological filtration mechanisms.

Sometimes, people can do it manually by removing the fish, cleaning the tank and refilling it with fresh water, and then returning the fish. Unfortunately, this might only be practical for small and simple tanks.

Moreover, removing the fish is traumatizing. So, if you are a beginner, why bother disturbing the fish every week. If you care about your aquarium, you don’t want to have neurotic fish.

Does a Filter Oxygenate the Aquarium Water?

Typically, stagnant water will have low levels of oxygen. But aquarium filters can go a long way in oxygenating the aquarium water, as they trigger water movement at the surface where oxygen exchange happens.

You can also add oxygen manually if there is a power outage or you do not have a battery-operated air pump. It is easy. You simply pour water into the tank from some height above. The higher you go, the better the oxygenation.

Do You Need to Change Water if You Have an Aquarium Filter?

The short answer is yes. You see, aquarium plants and filters will only remove waste to some extent, meaning that water changes are necessary. Ideally, you should change it weekly.

On top of this, try to rinse your filter media at least once a month.

Should You Turn Off an Aquarium Filter at Night?

As you already know, the aquarium filter plays a key role in maintaining the health of your tank. So, turning it off for several hours can eventually lead to problems.

Some beneficial colonies of bacteria that live in your tank may die. So, it is not a good idea to turn off the aquarium filter at night.

How Often to Clean an Aquarium Filter?

While the process of determining when and how you should clean your filter is not always cut and dry, try to clean it regularly. The routine you set up will most likely determine your cleaning schedule.

For small filters, you should clean them every week, while for bigger ones like canister filters, try to clean them every three months.


You must have had a good reason to keep an aquarium at your house. So, don’t let chemical and physical waste products interfere with your fish’s life. A healthy aquarium depends on clean water.

So, you should have a good filtration system. A good rule of thumb is to keep your water clean and your aquarium filter dirty. But having a filter doesn’t mean you will sit back and neglect your aquarium.

It will not do all the work for you. You will still need to maintain it and make sure it doesn’t get clogged.

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