How Long Does Carbon Last in an Aquarium Filter? (Explained)

Last Updated: July 11, 2022 by Flora Gibbins

When fish start dying in an aquarium, most people think the problem resides in the filter. Most times, people ask me “How long does carbon last in an aquarium filter?”, thinking that this is the root of their problem.

Contrary to manufacturer claims, there is no exact time-related answer for how long activated carbon will last in any given tank setup.

Overview

Throughout time, activated charcoal or activated carbon has been used to filter out all kinds of air and water-based impurities. In fact, activated charcoal is even used medicinally to help stop certain kinds of poisons from being absorbed by the stomach.

Activated carbon works well as a filter media because it has very small pores. These pores are big enough to let chemicals in, but they can’t easily get back out again. Over the years, activated carbon in relation to aquariums is something else my views have evolved on. 

Later on, as I began to explore black tanks, self-cleaning tanks, and heavier usage of live plants, I got further away from seeing activated carbon as an absolute essential. 

These days, I still keep carbon media on hand to manage emergencies. This includes if I suspect there are excess contaminants such as smoke or other toxins in the air that will find their way into the tank water.

Under these circumstances, the activated carbon can be a lifesaver for all creatures in the fish tank. 

Even if you have a heavily planted tank, they may not absorb toxins, or not do so quickly enough to prevent loss of life in the fish tank.

However I don’t use it in every fish tank, nor do I use it exclusively to manage various water quality concerns.

Nevertheless, knowing how long does carbon lasts in an aquarium filter is very important for optimizing its use.

Contaminants that Are and Aren’t Filtered Out By Activated Carbon

For the most part, the activated carbon filter media removes chlorine, tannins, and several other organic molecules from the water. It will also remove some fine debris from the water and sediment.

If you already use a freshwater aquarium conditioner, then you won’t need to be concerned about using activated carbon to remove chlorine or related compounds from tap water. 

Depending on the water type and aquatic inhabitants, you may have more of an interest in preserving tannins as opposed to removing them.

From these two perspectives alone, it becomes easier to see why you might not always want to use activated carbon exclusively for chemical filtration in a tank. 

Since granular activated carbon doesn’t remove ammonia, and also won’t reduce water hardness or molecules that contribute to pH fluctuations, it isn’t even of use from that perspective.

If you want tank water that appears crystal clear, it will be easier to achieve with activated carbon.

Even though sparkling clean water in a tank looks attractive the doesn’t mean the water is actually healthy. You may still have rapidly fluctuating pH levels, hardness issues, and other problems.

How to Get More out of Activated Carbon

There are 3 ways to get more out of activated carbon:

• Try to add as much activated carbon to the filter as possible. The more carbon you have, the longer it will take to fill up the pores.

• Make use of live plants in the tank that will absorb volatile organic compounds or other chemicals that would be trapped by activated carbon.

• Keep the bioload low. Do not overfeed fish or allow excess waste to build up in the tank. This will also help reduce the kinds of fine debris that are inclined to clog up activated carbon pores faster.

Substitutes for Activated Carbon

Choosing a substitute for activated carbon depends on what aspect you are interested in duplicating.

• To reduce cloudiness use filter floss or a denser filter sponge. 
• To reduce yellow or brown-tinged water color, you can use zeolites. Just remember zeolites also remove ammonia. If you have too much zeolite in the tank, you can disrupt the nitrifying bacteria cycle.
• You can also prevent the water from yellowing in the first place by not leaving excess food or waste in the tank. Scavenger, or bottom-feeding fish, freshwater shrimp, snails, and algae eaters can all help with this.
• To remove odors from the water add more live plants. Hornworts, aponogetons, and anubias all work well for this purpose.

Why Activated Carbon Must be Replaced

When you are washing dishes, you always have to squeeze the sponge to make room for more water, detergent, and debris from the dishes.

Activated carbon is the same way.

Once the pores are filled up, there is no more room to hold more contaminants.

Unlike a sponge, you can’t simply squeeze activated carbon and make more room available. Likewise, rinsing the carbon won’t remove the deposits.

Instead, you will have to replace the activated carbon with new material.  It is easier and cheaper to buy activated carbon as opposed to making it.

But then let’s see how to make it, in this video:

Common Signs You Need to Replace Activated Carbon

There are three main signs that you need to change the activated carbon in your filter.

One of the first things you will notice is that the water isn’t as clear as it used to be.

If you have fish that routinely dig in the substrate, you will also notice that it takes longer for the water to become clear of ultra-fine debris.

Second, you will notice a change in the odor of the tank water. Normally, tank water should have no odor. 

When activated carbon is filled up, the water will take on an odor similar to corn tassels. It still won’t smell horribly foul or carry through the room. Nevertheless, there will be an odor present that will become more earthy as opposed to plant-like.

If the water smells foul, then you are more likely to have a sudden ammonia surge as opposed to a problem with the activated carbon.

Third, the water will begin to take on a yellowish, orange, or brownish hue. The longer you wait to change the activated carbon, the darker the water will get.

Depending on the tank setup and additives, this may indicate tannins are building up. If you are trying to keep soft water, acid-loving fish, then this is actually far more ideal than sparkling clear water.

 In this situation, activated carbon may clear the water up, but it won’t last for long. You will still need to gradually vacuum debris from the gravel and use live plants to absorb as much nitrate as possible. 

If the sludge is too heavy, you may also need to use aquarium-safe bacterial additives that break it down faster. 

Fun Fact: You have the option to use an ordinary filtering medium (i.e. sponge). The cleaning process is simpler. Know the basics on how to clean aquarium filter sponge to prevent diseases from contaminating your aquarium water and harming your pet fish!

Frequently Asked Questions

1. Can you recharge activated carbon used in an aquarium?

When you do this, however, the activated carbon won’t work as well. It is also usually considered more expensive than it is worth.

You might be better served learning how to make your own activated carbon. While this process isn’t especially difficult, you will need a reliable source of wood or other fibrous material. 

2. What is the longest amount of time activated carbon will last in an aquarium?

Activated carbon may last only a few days or as much as 2 – 3 months. 

The amount of time depends on many factors. If the tank has a high bioload or a lot of fish that dig in the substrate, the tank water may start to appear hazy in just a few days.

Similarly, if you don’t have live plants in the tank, or overfeed the fish, the carbon will need to be changed out within just a week or two to avoid odor and color problems.

3. Can you operate an aquarium filter without activated carbon?

Yes, any aquarium filter can be operated without the use of activated carbon.

The question is how well it will function. In the absence of activated carbon, you are likely to use something else in its place.

HOB filters can be harder to adjust in terms of media because of their design and the risk of flooding. 

For example, if you decide to use a denser sponge for filtering out debris, you may have to rinse it every few days to avoid water spilling out of the filter. 

In a similar fashion, if you decide to use porous ceramic or other media, it should not impede the flow of water through the filter to the point where a malfunction occurs.

 If you decide to eliminate activated carbon from a cartridge-based HOB, it will be best to also try a different filter design. 

Bubble-up filters can be packed with just about any kind of media without impairment in the water flow. Canister filters are also far more forgiving in terms of media adjustments than HOBs.

Conclusion

“How Long Does Carbon Last in an Aquarium Filter?” is an important question to ask if you are progressing past the first stages of aquarium care.

Once you know what activated carbon does, you will soon realize that there are better ways to manage aquarium water that don’t revolve around a constant focus on activated carbon.