What are the 3 Stages of Aquarium Filtration?

Stages of Aquarium Filtration

When you have fish or turtles in a tank, it’s crucial that you do the best you can to make sure that the water remains clean and healthy for your pets. If you don’t, then the water can become dirty and unsightly. It can actually become so polluted that your fish and turtles can die.

The problem is that water generally just becomes dirtier in your fish tank as time passes. This is understandable when you realize that these animals just excrete in the water, and where else will the fecal matter go but float around polluting the water? The fish may also not eat every bit of food you put in, and the uneaten food can turn into ammonia which makes for a highly toxic environment for your marine animals. It’s also common for dust to settle into the water, and sometimes your water may have too much chlorine in it.

So what can you do? Every fish tank owner knows that it’s imperative to use the proper filtration system. In general, these aquarium filtration systems have several layers of filter media that can sift the water and take out various types of contaminants. Mechanical and biological filtration layers are very common, but many others also include chemical filtration to really keep the water crystal clean. Together, these 3 stages of aquarium filtration can make sure your fish and turtles remain healthy while you enjoy the clear sight of them swimming around.

3 Stages of Aquarium Filtration:

1. Mechanical:

This is also sometimes called the physical filtration stage, and that’s because it acts like the filter for your coffee machine or for your bathroom drain. It physically traps particles so that the water that passes through is cleaner. The filter doesn’t change the chemical makeup of the water at all.

These filters have holes of various sizes, which determine the kind of particles they trap. The larger holes in very coarse filters can trap larger particles, but let smaller contaminants pass through. The fine and extra fine filters can also trap even the smallest particles, but they can trap too many particles quickly because they also block larger particles.

The most efficient system starts by passing the water into the filters with the largest holes. Then the succeeding layers have increasingly smaller pores so that the fine filters won’t have to deal with the largest particles. These filters need to be cleaned regularly, but some of the particles they hold may not be released. If that happens, you need to replace the filters.

2. Biological

This is the filter media that houses the growth of beneficial bacteria. This type of filtration media is almost always part of the aquarium filtration system because it works hand in hand with mechanical filters.

One of the main problems with aquarium tanks is ammonia, which is decidedly lethal for fish and other aquatic animals. Even a tiny bit such as 2 parts per million can kill your fish. Ammonia comes from organic matter such as fecal matter and uneaten food, and these contaminants are ever present in the water.

It’s this dangerous compound that makes the beneficial bacteria so, well, beneficial. That’s because bacteria are living organisms, and they just so happen to eat ammonia. As they consume the ammonia, they reproduce very quickly. This means the whole process can result in quickly reducing the ammonia levels to zero.

Of course, the ammonia doesn’t disappear entirely. The bacteria eat the ammonia and then these tiny organisms “excrete” nitrite. The nitrite is still dangerous for the fish, but fortunately, they’re not as lethal as ammonia. Eventually, though, the nitrite level can be too much. This is why you still need to replace the water in your aquarium every now and then even when you have a very efficient aquarium filtration system.

Even small biological media can last for a couple of years. There are many types of these to choose from. You can use lava rocks, gravel, or even plastic media. All these feature a surface area where the beneficial bacteria can grow and thrive.

3. Chemical:

This works like the biological filtration system, as it changes dangerous contaminants into more benign components. It’s just that this time you have carbon or resins that chemically change the nature of these contaminants.

Activated carbon is a very popular option for chemical filtration. It can take care of organic contaminants that your mechanical and biological filters can’t handle. The carbon has microscopic pores to which various organic or inorganic matter can stick. This often involves a process called “adsorption”, when the atoms, molecules, or ions of the contaminants adhere to the surface of the carbon.

With activated carbon, discolorants can be removed from the water so you can enjoy crystal-clear water. There is a limit, however, as to the efficiency of the pores. Once the carbon pores are all filled, the activated carbon won’t work anymore and you need a new one.

Resins (or more specifically, ion exchange resins) are another option, and this is becoming increasingly popular. The resins can attract a particular molecule to adhere to them. This means this can be used to attract ammonia, nitrate, or dissolved organic matter. Resins can be used alongside activated carbon to strengthen chemical filtration, and they also help with biological filtration.


Technically, you can have more than 3 layers of aquarium filtration. After all, your mechanical filtration system can include 3 or more layers that filter out particles of various sizes efficiently. Add the biological and chemical layers, and you can easily find aquarium filter systems with 10 or more layers altogether.

Still, you have 3 basic filtration systems to use, and it’s highly recommended that you make use of all of them. Each one has its benefits, and working together can make very sure that your water is healthy and clean. Include the mechanical, biological, and chemical filter media, and you can be much surer that you’re doing all that you can maintain a healthy environment for marine pets.

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