Where to Put Activated Carbon in Canister Filter?

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The filtration system represents the aquarium’s lungs and kidneys, as it improves oxygenation, filters chemicals and debris, and cleanses the water. Not all filtration systems work based on activated carbon, given that they target specific goals.

Biological filters only work against bacteria, viruses, fungi, and other pathogens, while mechanical filters are only good for removing debris, dirt, plant matter, and other residues. Activated carbon-based filters are chemical systems designed to cleanse the water of chemicals and other contaminants that could affect the tank life.

Today’s article is meant to assist people who’ve never used activated carbon for their filtration system before. While the filtration system is relatively easy to use, adding activated carbon into the mix changes things a bit. So, let’s dive into that.

What is Activated Carbon Used For?

Activated carbon is a carbon-based substance (right?) treated under extreme temperatures to alter its surface composition. The material displays a micro-porous surface very well visible on intact blocks of activated carbon. The material also comes in the form of powder, depending on how you plan to use it.

The activated carbon’s main use is chemical decontamination. Biological and chemical filters will only remove bacteria, viruses, and debris, and dirt, respectively. But they cannot remove chemical contaminants due to their particles being too small. This is where the activated carbon filter comes in.

Activated carbon-based filters provide effective chemical filtration, cleaning, and protecting the water from a variety of contaminants. Such a filtration system may sound redundant to many aquarists. After all, there’s no way you can get any chemicals in your tank since you’re so careful about it, right?

Unfortunately, your tank water can become contaminated via a variety of means. More inexperienced aquarists face such problems more often, as you might have suspected. Here are some sources of tank water contamination to consider:

– Chlorine Poisoning

Chlorine is a sterilizing agent with a variety of uses in the real world. These include disinfection and deodorization, which already recommend chlorine as ideal for the sanitation system, among other things. Chlorine is safe for humans within certain parameters and is used to sterilize the regular tap water you’re getting in your home.

The problem is that chlorine is toxic to fish and plants. How does chlorine reach your tank water? Inadequate water changes are most often the culprit, with people using tap water in the process completely oblivious to the risks.

Your activated carbon-based filter will eliminate chlorine and other chloramines, eliminating the risk of chlorine poisoning. Even so, you should always dechlorinate the water before using it for your water change. Boiling it or using a water conditioner can solve the problem fairly fast. Always check the water for any traces of chlorine before use.

Or you can simply use other types of waters when performing a water change, like distilled or RO water. Just remember that these require re-mineralization before use since they are devoid of nutrients.

– Contaminated Decorations

One of the most widespread causes of water contamination is the introduction of contaminated plants, rocks, or tank decorations. Plants are some of the main culprits in this sense since they often come with a variety of hitchhikers, including snails, snail and worm eggs, parasites like Ich, and even chemical components.

Adding wild plants to your aquarium will increase the risk of contamination, depending on where you got your plant.

Feeding your fish wild worms and crustaceans will produce a similar effect. Wild worms are generally filled with parasites or chemicals that they’ve absorbed from their surrounding environment.

Tank rocks and personalized decorations are also potential vehicles for a multitude of chemical agents. This is why you should always sterilize anything you add to your tank before adding it.

Some items may even necessitate quarantine for the same purpose. Finally, a good filtration system based on activated carbon will provide additional assistance as an extra layer of defense.

– Ammonia Outbreak

Ammonia is the normal result of decomposing organic matter. This can come in the form of dead plants and leaves, fish food residues, fish waste, dead fish, disturbances in the tank’s biofilm, etc. A properly cycled aquarium will manage ammonia and nitrites effectively, as the tank’s cultures of beneficial bacteria will break these chemicals down and turn them into nitrates.

You can then perform regular water changes to further dilute the existent ammonia, which, along with a robust filtration unit, will keep the environment safe.

The problem is that, sometimes, ammonia levels can spike seemingly out of nowhere. This happens more often in aquariums with sandy substrates. Sand is great for tanks since it’s compact and dense, preventing dirt and waste from sinking in. This makes the substrate easier to clean.

The problem is the danger of anaerobic pockets which form under the sand due to the sand’s compact nature. Water cannot circulate through the substrate, leading to anaerobic pockets where bad bacteria spread and produce ammonia intensively. These ammonia-filled pockets can burst due to plant growth or bottom-dweller activity, releasing the chemical into the open water.

Decomposing bodies of dead fish make for another source of ammonia burst. It’s easy to overlook the death of one or 2 fish in an overpopulated and overplanted tank. This will cause the bodies to decay in the water, releasing ammonia at an accelerated rate.

At this point, having an activated carbon filter will literally save lives.

What Order Does Activated Carbon Go in Canister Filter?

You should place your activated carbon in your canister filter’s last tray (filter media). The reason for that is simple. Your activated carbon has a porous surface that traps any chemical micro-components passing through. The material is extremely fine, allowing it to catch even the smallest particles.

The problem is that the debris and dirt present in the filtration system can quickly render your activated carbon useless. The matter will quickly fill up the carbon’s porosity, decreasing its effectiveness and even disabling it entirely.

So, your activated carbon needs to remain safe from the rest of the filter media that will trap larger particles and muck. Only the finer, microscopical chemical grains will get through, and the activated carbon will be there to catch them.

How Much Activated Carbon Should I Put in My Filter?

This all depends on your tank’s size, other filtration systems, and how contaminated the environment is. The general recommendation is to use 2 cups of activated carbon for every 50-55 gallons available. You may need to use less if your regular filter is already doing a good enough job.

How Often to Change Activated Carbon Filter?

You should change your activated carbon every 3-4 weeks at most. The activated carbon will deactivate past this point. The notion of deactivation refers to the activated carbon reaching maximum physical absorption. So, it can no longer take in any more particles, becoming inert in the process.

Unfortunately, there’s no way to reverse the process. The only option you have left at this point is complete replacement.

Can You Clean Activated Carbon Filter?

No, you cannot clean your activated carbon filter. Once your activated carbon loses its efficiency, it’s time to throw it away and replace it completely. You can tell that the activated carbon isn’t doing its job anymore by checking the water quality. If it emanates a nasty, stale smell, or appears to change its color, consider that your activated carbon may have become inactive.

Since we’re here, we should touch upon some widespread myths regarding activated carbon as well:

  • Reactivation – You will see numerous guides claiming to help you reactivate your carbon. Some of these guides refer to placing your activated carbon in your oven at a higher temperature, supposedly designed to remove all of the chemicals and clean the material. This is false. While it’s true that high temperatures and pressure will rearrange the material’s molecules for a full reset, you can’t reach those values in your home oven. Unless you have a professional piece capable of delivering industrial-grade temperature, which you don’t.
  • Substance leakage – This phenomenon refers to the activated carbon spilling some of the trapped toxins back into the environment once it has reached its maximum lifespan. This is, naturally, false. Activated carbon doesn’t leak any of the trapped chemicals back into the environment, but it will leak phosphates, though. However, this only happens if the activated carbon has already been contaminated with phosphates during the production process. The product’s label should state this clearly if that’s the case.

So, once your activated carbon’s lifespan has reached its end, replace the material altogether.


Contrary to what you’ve read so far, you don’t absolutely need activated carbon. In fact, most aquarists don’t since they can rely on their normal filtration system to get the job done. Activated carbon is only necessary in case of chemical contamination or if you know for a fact that there’s a high risk for such issues.

Other than that, perform regular cleaning, avoid overfeeding, remove fish residues and fish waste, and have live plants in the habitat and you’ll be fine. Just keep in mind that activated carbon is a reliable option that you can fall back on should the need arise.

avatar 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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