Why And How To Use Activated Carbon In A Fish Tank

When I first started with my fish tank a long time ago, I was recommended to buy activated carbon for my fish tank. I was wondering what it was for so I started researching.

Activated carbon is a very porous charcoal. It helps with removing medication and to remove pollutants from the water in your fish tank. Activated carbon lasts for typically 1 month depending on how much pollutants in the water.

I will go through what activated carbon is, what it’s used for and how to use it. Yes, just about everything you need to know about this good-to-have product!

What is activated carbon

Carbon is an element that you easily find on our planet. It’s a building stone in nature. When you burn something it turns black. It’s carbon and is referred to as charcoal.

Activated carbon is basically that charcoal but with a lot of small pores. Those pores are created when the producer is creating the carbon. What makes these pores necessary is that it traps pollutants and rests from medication.

Activated carbon is a very good absorbent and is used for a lot of different things, not just in fish tanks. Other uses are in the industry and medicine for example.

Since the carbon absorbs pollutants it’s a good way to remove bad smell from the tank, now the bad smell is a sign of an unhealthy tank and you should go to the bottom with the source of the smell rather than masking it. carbon could be used to remove the smell while you are searching for the source.

Activated carbon comes in a lot of different forms. It could be made from, for example, peat, wood or coal.

All these options are viable. When buying activated charcoal from the pet store you could be safe it’s made for fish tanks and won’t ruin anything for you (even though it’s hard to ruin anything regarding activated carbon).

What you shouldn’t do is making it yourself. You have to process the charcoal to get the pores for it to be effective to remove pollutants. It’s nothing you should or could do at home.

If you have a lump of charcoal after barbequing you should not put it in your fish tank and think it will do anything. It will just sit there in your filter and do nothing. It has to be activated first so be sure to get the stuff from the pet store and you could be sure the coal will do something for you when you put it in.

Also, don’t buy the coal second hand. You never know if the coal is saturated or if it really is activated carbon.

What doesn’t it remove

Activated carbon isn’t this miracle that will remove everything and everything will be fine. What it doesn’t remove is one of the most important things to keep track of in a fish tank.

It’s ammonia, nitrite, and nitrate. These pollutants are lethal for your fish and they are the nitrate cycle. It starts from the fish waste and it’s the bacteria in your fish tank that will break those down and make them safe for your fish.

Don’t expect to get control over an ammonia spike just by putting in activated carbon. Instead, try to help your bacteria by putting in bacteria starter and remove the ammonia by doing frequent water changes.

What activated carbon doesn’t remove either is heavy metals from the water which you might bring into your tank during a water change.

You shouldn’t have any problems with it since that’s why you use a water conditioner. What activated carbon is removing from the water is chlorine thou but that’s a job for the water conditioner in my opinion. It does the same job of removing chlorine.

When you are putting it in, let it do the work. Don’t put in activated carbon just to remove chlorine.

How to choose the right type of carbon

As stated before, activated carbon is used for many other things than just fish tanks. It’s therefore important to choose the most effective for fish tanks in particular. I have written an article about which carbon I use and which I recommend.

What you look for when choosing the perfect activated carbon is mainly the pore size and which classification the activate carbon has.

Activated carbon comes in different types. Those are:

  • Granular activated carbon
  • Bead activated carbon
  • Powdered activated carbon
  • Extruded activated carbon

The most common in water cleaning is the granular activated carbon. It’s very good at absorbing gases and vapors which is exactly what we are after when we put in activated carbon in our fish tank filters.

While the other classifications probably will work, they will not work as efficient as the granulated activated carbon will.

Another thing to have in consideration is the ash content in the activated carbon. For a fish tank, it’s important the activated carbon has low soluble ash content. That’s because the high content of ash in the carbon is making iron oxide leach out into the water and promote algae growth.

It will also risk heavy metal poisoning in your fish because of that.

Another thing to be aware of is phosphate. Some activated carbon is containing phosphate and, while it won’t hurt your fish directly, the phosphate will make algae bloom and it’s not nice to look at and could be hard to remove sometimes.

When getting activated carbon, get it from the pet store. Those products are made for a fish tank and you know you will get something that’s not going to hurt your fish tank.

Don’t buy the coal from someone else. You don’t know if it’s been saturated or if they have made it themselves. You don’t want the risk of the carbon not working when you really need it. It’s not worth it just to save some money!

Activated carbon is not something you have to optimize. You just need it to work. As long as it’s activated carbon that’s not saturated it’s fine. You need to have it in your cabinet in case you need to use it. For example, when medicating your fish, to stop the medication you need activated carbon. Don’t waste too much time searching for the perfect carbon.

Medication and activated carbon

One typical case where you need activated carbon is when you need to medicate your fish. You don’t use it during the medication. You use it when you want to stop the medication.

It’s crucial to have some activate carbon ready to be used for that reason. You never know when your fish are getting sick and it’s always, for some reason, in the worst possible time.

Medicating your fish will usually go over some days and you will have time to get carbon. I would still suggest you have the activated carbon ready since you will have a lot of things to think about as it is when your fish is ill.

Why do you want to stop the medication with activated carbon? Why no lite the medication be and be removed during water changes?

The medicine is killing bacteria and parasites that are infecting your fish. It’s also killing the bacteria you want in your fish tank for the nitrate cycle to be working.

When you have medicated as the instructions for the medicine tells you, you are done with it. If you keep going with the medication by just letting it be in the fish tank, it will kill your good bacteria in the tank and you run the risk of an ammonia spike which is lethal to your fish.

It will also put more stress on your fish and stress is the most common cause of dead fish in a fish tank.

By stopping the medication in time, you will do as little harm as possible to your good bacteria, which leads to a lower risk of a lethal ammonia spike. It also leads to less stress for your fish.

Just be sure not to have activated carbon in your fish tank while medicating.

How to use activated carbon

There are some steps you have to do while using activated carbon.

First, you have to clean your filter. Don’t do it in the tap water since the chlorine will kill the bacteria. Instead, clean it in the water you remove from the fish tank while doing a water change. That way you won’t lose bacteria and you will lean your filter from debris.

Why you have to do that is because if the activated carbon is exposed to debris it will lose its effectiveness a lot quicker and you have to change it more frequently. By cleaning the filter first, you will remove a lot of debris already collected and it will make the mechanical filter more effective.

When your fish tank filter is clean, put your carbon in a filter bag or a tight. That’s for holding the carbon in place so it’s easy to put in and take out from your filter. Then rinse the carbon in warm water.

To know how much carbon you put in, you read the package. I use Sera Super Carbon and the instructions say to put 10 g (0.4 oz) fo up to 20 l (5.3 gallons). That’s a good rule of thumb and I would imagine other brands have about the same dosage.

And that’s right you put it in the filter. Not behind a rock or in the substrate. You want the water to flow through the carbon and also to not get debris in the carbon as you will if you just put it in the tank.

To not expose the carbon to debris put it behind the mechanical filtration. That way the mechanical filtration will take care of the dead leaves and the carbon will take care of medication rests and other pollutants.

When not using the activated carbon, the package should be stored in a dry place for it to not absorb any pollutants when you just store it.

How long to have activated carbon in the filter?

How long to have the carbon in the filter is determined by the pollution and how much medicine you have given your fish. It’s also determined by if the carbon is exposed to debris or not. Hence the placement after the mechanical filtration and not before.

When following the directions on the package, and with a placement behind the mechanical filtration and “normal” pollution the activated carbon will normally last for about one month.

When the activated carbon is saturated, which is impossible to see, it’s just to throw it away. You can not re-activate it as long as you are not cooking with an extremely hot furnace in your kitchen.

If it’s the case, it will still not be worth re-activating the carbon since the energy cost to do it will probably be higher then what it will cost you to buy new activated carbon.

Also, by not using your extremely hot furnace for cooking, you will not run the risk of it being ineffective when you use ur “re-activated” carbon.


In conclusion, you should have a package of activated carbon in your fish cabinet. It’s mainly because of stop medicating the fish. You never know when your fish will be sick.

You don’t have to search for the perfect carbon, just buy it from the pet store made for fish tanks and you can be sure you will get a product that will work and do what it promises.


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

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