Aquarium Filter Media – Complete Guide

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If you own an aquarium, you most likely also have a water filter installed beside it, to clean the water for your fish. No matter what type of water filter you’re using, pretty much every modern filter will clean your water using 3 different filtration stages:

  • Mechanical filtration
  • Chemical filtration
  • Biological filtration

In this article you’ll learn about all these 3 filtration stages, and why you need to have them installed in your device. At each section, I will cover what the filter media does, how to maintain it and I will also recommend one that works for most people, no matter the filter in use.

Before we jump into the mix, you should know that all three types of filter media work in sync to provide a crystal clean water environment for you fish. The most important stage is the biological filtration, which is responsible for removing ammonia and nitrites from the water column.

Every media takes care of different residues in the water. So, skipping one or the other will result in poor water quality and eventually death of your live stock.

When you buy a filter make sure it includes all 3 filter media. Usually, all modern filters come with these systems pre-installed. But in any case, check before, so you won’t get any surprises.

Now, let’s start with the first media, which is also the first filtration stage in almost all filters:

Mechanical Filter Media

What is mechanical filtration? This is the first stage of filtration in most devices. What does it do? Mostly a mechanical filter media will get rid of physical debris that floats inside the aquarium’s water.

A mechanical filter media will capture all the solid particles (mostly visible) that float in the water. These can include solid waste that remain from decaying plants, uneaten fish food or fecal waste from the fish and other marine life in the tank.

How does a mechanical filter do the job? When the water goes inside the filter, it will be pumped through the mechanical media. There, the media will capture al the solid waste, and only let the clean water flow through.

What happens with the waste? In most cases, it will gather on the mechanical media. That’s why every mechanical media needs some maintenance work. How can you clean the media in this case? Well, it’s actually pretty simple.

Just take out the media from the filter, and run a constant stream of clean water through it. All the remains will be removed from it this way.

However, there is a time when the media has to be replaced. When does this occur? Usually, when it gets too old. In most cases, this is quite noticeable. But, if you don’t know exactly when is time to replace the media, just take a look at the user manual of your filter.

There you will find the exact time period a media is supposed to be used. If the period isn’t present in the document, just make a quick online search, and you’ll find your answer.

Sponge mechanical media will work years, if you clean them regularly, like described above. Filter pads should be replaced more often, because due to their high density, it is almost impossible for the dirt to be washed out.

Now, let me tell you about a good mechanical media that works for most people. Before that though, you should know that even you can create a mechanical filter by yourself. How?

With a simple piece of sponge that you find at home. Of course, make sure it’s totally clean. You would need a piece that isn’t very thick, about 2 or 3 cm thick. Cut it to the size of the available space where it fits and put it inside. That’s it! You created your very own mechanical filter media.

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But if you don’t have enough sponge at home and need something that is specially designed for water filtration, I highly recommend a foam filter media pack from Amazon. You can get a lot of sponges at a good price. A pack will last you for years.

Usually a set contain 3 different types of foams: coarse, medium and fine.

The coarse foam will filter out bigger debris, while the medium will catch the smaller particles and finally the fine foam will filter out the fine particles. A filter pad can also be placed after the fine, to have a crystal clear water.

The materials are very durable, so you can clean it many times before purchasing a replacement.

Biological Filter Media

This is the second stage of filtration used by any modern aquarium water filter. What does it do? Let’s see about this first:

A biological filter media consists in cleaning the water from nitrification residues. More exactly, every aquarium will contain ammonia and nitrates that fishes leave behind. Why do you need to get rid of these substances?

Because any of them in larger quantities will sicken your fish and possibly other marine life as well. I bet you don’t want that to happen. Actually, if these substances grow to be too much, certain fish breeds can even die from over-exposure.

This is where the bio-filter media comes into play. A biological filter media is basically a filtration system that filters the water from ammonia and nitrates. How can it achieve this?

Every bio-media is made up of bacteria colonies. You didn’t expect that, huh? Simply put, a bio-media in a container, in which the bacteria colonies will live. These bacteria must consume ammonia and nitrates to stay alive.

So, putting them inside the fish tank, will help you clean the water from these poisonous particles.

How can you install and maintain this media? It’s quite simple. Usually, the filter device includes bio-media as well. If you buy such a filter, you don’t have to do much, because everything is pre-set.

But in other cases, you might have to buy the bio-media yourself. When you have it, just place the media inside the container chamber pre-destined for this type of filtration.

The most important thing you must check all the time is for your bacteria colonies to not die out. After all, they do all the job. It’s important that you do everything as prescribed by the manufacturer of the bio-media that you bought.

If you don’t do all the steps written in there, you could put in danger the whole bacteria colony, and they can die out quickly. So, make sure you do everything as needed.

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A great biological filter media is the following one: Fluval Biomax Filter Media. Just like before, you can find this on Amazon too. What does it include?

This product includes 500g of bio rings, that comes with a complex pore system where the bacteria colonies can thrive and live longer. It’s good to know that you can use this media with both salt and fresh water as well.

If you have sensitive fish in the aquarium, you can be sure it won’t affect their well-being. The product was designed to not affect the environment in any negative way. The only goal of this media is to remove the nitrates and ammonia from the water, so your fish can thrive and live as long as possible.

Now that you know how bio-media works, and why it’s so important, it’s time we talk about the last filtration media, that must be present in all aquariums:

Chemical Filtration Media

Chemical filtration is the last resort when it comes to having a crystal clean water environment for your fish. Mechanical filtration will get rid of solid particles in the water, while biological filtration will remove the built-up ammonia and nitrates that can harm your fish. So, what does chemical filtration do?

Chemical filtration will remove all the dissolved organic substances that remain in the tank after the biological filtration is completed. What kind of organics are removed?

For instance, the compounds that contain phosphates are dissolved. These are important algae nutrients, and if they’re present in the water, they can prevent the growth of certain corals.

Also, if too much dissolved organic compounds are in the aquarium, the water will gain a yellowish look. It will feel like a dirty environment. But not just that. If you don’t clean the water from this, light is going to be prevented from easily entering the fish tank. Coral growth is also affected by this.

The idea is to make sure you have a chemical media filter put in place for taking care of these problems. How can you install this media? Let’s see about that right now:

The installation of the chemical media is very similar to the bio-media set-up. You can do this by placing the chemical media inside a power filter, where the water is pumped powerfully through the media. A strong flow of water is desired with most chemical media filters.

But there is another way to handle this as well. The media could be put inside a simple bag, that lets water flow through it. The bag has to be placed in a silent, discreet spot inside the tank. This way, the water will flow through the media, but it won’t be pumped strongly.

This method is better if you want a steady and continuous filtration, that happens when most of the organic residue is already removed.

How is the maintenance handled? Differently from biological filtration, chemical filtration won’t transform those substance into less harmful ones. The particles will be bound to the chemical media, thus not letting it flow through the water freely.

Because this is the case, chemical media has to be replaced more often than biological media. This is the best way to ensure a proper water filtration chemically. The replacement period is probably written inside the user manual, or on the box.

But you can notice the process by yourself. When nothing else, or almost nothing is binding to the media, it’s probably time to replace it with a new one.

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A product that works for most people is the Chemi-Pure media. You can get it on Amazon in 2 different sized boxes (5 and 10 ounces). This product is designed to remove the waste that’s left after the biological filtration, like the nitrogenous waste.

It helps keep the water pH level at a safe range for your fish, and it can also help your fish live longer. It works in both salt and fresh water environments.

A box that contains 5 ounces of material is enough to filter a 20-gallon fish tank, and the bigger 10 ounce box for 40 gallons of water.


Now you know the importance and the job of each filter media. It is also important that you set up the filter media in the right order. When setting up your filter, make sure that the water hits first the mechanical filtration, then flows through the biological media and finally is purified with the chemical media.

Setting it up the wrong way, you will have a lot of issues.

Also keep in mind, that although some filters come with all 3 filter media types, those are setup wrongly by the manufacturer. It is obvious that most filter manufacturer wants to make more money off you and try to make you replace filter media often.

Setting up the filter media in your filter as I described in this article, you will not have to replace filter media too often. In fact sponges and biological media should never be replaced (just in very rare cases), while activated carbon should be replaced 2-4 times a year.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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