Carbon Filtration In Freshwater Aquariums

Written by Kmuda. Posted in All About the Water

Use of carbon filtration in modern freshwater aquariums has become a heated topic of debate. The purpose of this article is to provide some details as to the use of carbon, allowing the individual aquarist to decide if carbon filtration is worth the expense and often-repeated hazards.

As an initiator, let me be clear in stating that carbon use is optional. It is by no means necessary. I currently have tanks in which carbon is in use and I have tanks in which carbon is not in use. There is no discernible difference in water quality or clarity between them. The question is not is carbon is necessary, but rather is carbon beneficial.

Let’s begin this discussion by detailing the Pro’s and Con’s of Carbon:


Pro #1: Carbon Removes Dissolved Organics and Toxins:

Activated carbon removes contaminants through two methods, adsorption and catalytic reduction (negative ions in the contaminant are drawn to the positive ions in the carbon). In general, organic particles are removed via absorption while chemical contaminants are removed via catalytic reduction.

Of the organic substances removed by carbon, the two primary categories are tannins and phenols. Tannins discolor the water and phenols give the tank a fishy smell.

Carbon will also remove toxins such as pesticides, tar and nicotine (from cigarette or cigar smoke), detergents, hydrogen sulphide, copper, arsenic, bleach, and host of other contaminants.

Additional details on exactly what carbon removes are identified in the below table:

Pro #2: Use of Carbon makes bio-filtration more effective.

This is based upon carbon's role in removing dissolved organics. Dissolved organics are a food source for heterotrophic bacteria, which can in turn out compete nitrifying bacteria for space and oxygen. The sole limiting factor affecting heterotrophic bacterial growth is the availability of organics. Since carbon removes many dissolved organics, it helps limit the colonies of heterotrophic bacteria.

Autotrophic nitrifying bacteria can double every 12 to 33 hours. Heterotrophic bacteria can divide every 20 minutes.

Heterotrophic bacteria have their place in an aquarium as their primary function is the initial breakdown of waste, which is one initiator of the nitrogen cycle. Carbon’s adsorption ability removes excess dissolved organics, which would otherwise serve as a food source resulting in higher populations of heterotrophic bacteria, which in turn may result in a decreased population of autotrophic nitrifying bacteria.

Pro #4: Carbon removes medications:

Even those who do not use carbon for anything other than this purpose accept this as fact. It need not be discussed further.


Con #1: Carbons usage life is very short

This is true. Under most conditions, carbon has a functional life of one to two weeks, perhaps even shorter. The average functional life of carbon is difficult to determine as many variables apply, such as the amount of dissolve organics in the water, the effectiveness of mechanical filtration, and the amount of carbon in use.

A safe assumption is that carbon’s functional life in most tanks will not exceed 2 weeks. With this assumption, a best practice is to replace half of your carbon weekly. This will result in fresh carbon within the tank for each week with no carbon remaining in the tank for longer than two weeks.

Con #2: Carbon removes trace elements:

A commonly referenced hazard associated with carbon use is that it removes trace elements. Carbons ability to remove trace elements (or minor elements) is more myth than fact. Carbon will remove trace and minor elements if they are in a specific form, which is pH dependent. For most elements, the pH must either be below 4 or above 10 for them to be in a chemical form that can be adsorbed by carbon.

However, it is accepted that carbon will adsorb iron in the ferric state.

The general consensus is that while carbon may have a negative affect on planted aquariums by removing some minor elements, it is actually inefficient at this process. For fish only aquariums, regular water changes will sufficiently maintain necessary levels of trace elements. For planted aquarium, if you are concerned, trace elements can be supplemented through commercially available additives.

Con #3: Carbon leaches phosphates

This is partially true. All carbons will release some amount of phosphate but some inexpensive carbons are acid washed in phosphoric acid and these carbons will leach a substantial amount of phosphate into the water. However, all carbons from reputable manufacturers are currently acid washed with hydrochloric or sulfuric acids. These will leach very little, if any, phosphate. Provided you are not a reef aquarist (and since this article is about carbon use in a freshwater tank, that is unlikely) the amount of phosphate leached is less relevant. Suffice it say, buy inexpensive bulk carbon and your phosphate levels may increase dramatically. Buy more expensive name brand carbons, phosphate concerns are minimal.

Regardless, phosphate concerns can be eliminated by soaking activated carbon in a saltwater solution for a few days prior to adding it to your aquarium.

Con/Myth #4: Old carbon will leach organics back into the water (de-absorption)

This is true in industry, but not in our aquarium. Carbon is widely used in industrial settings to recycle precious metals. Industrial use of carbon involves the capturing of a specific substance at one pH extreme (below 4 or above 10) and then reclaiming the substance by converting to the other pH extreme. If a pH shift of this magnitude occurs in an aquarium, carbon leaching organics back into the water is the least of our worries.

Con #5: Carbon may cause HITH

This is a controversial subject. There are many reported instances of HITH going into remission in conjunction with carbon removal. I happen to be one of those cases. As a result, regardless of my opinion on the benefits of carbon, I do not use carbon in my Oscar tank. Recent studies of Marine Fish have identified a relationship between HITH and carbon dust.

Con #6: Carbon may mask real issues:

As an example, a dead rotting fish in a tank will normally cause the odor of the water to change. In the absence of carbon this odor may be detected by the aquarist who may then search out the cause, find the dead fish, and remove it from the tank. If carbon is in use, the odor of the water may not change, and the dead fish may be left to decay potentially resulting in problems far more significant than a smelly tank.

Con #7: Carbon serves no useful function beyond the removal of medications when needed.

Well, that is for you to decide after reading this article.


So, let’s say you’ve decided that carbon use is not for you. No problem, no need to read further. However, if you decide you wish to use carbon then there are certain questions you may ask:

What type of carbon should I use?

Bituminous coal based carbons are the preferred carbon for use in an aquarium. Coconut shell, lignite coal, wood, and peat based carbons are not as effective. If the carbon is not labeled specifically as bituminous coal activated carbon then skip it and find a product that is labeled as such. In addition, there are several guides to selecting the best carbon:

  1. Carbon that is shiny and/or dusty is usually inferior in quality
  2. If the label provides details such as ash content, porosity, and phosphate content, then chances are it is from a reputable manufacturer and is a quality carbon.
  3. Compare weight and volume. The greater the porosity, the better the carbon. So if selecting carbon of equal volumes, select the carbon with the least weight. Carbon should be so porous that individual granules may actually float.
  4. In general, smaller granules are preferable to larger ones.
  5. Carbon should be as free from ash as possible
  6. Carbon should be “acid washed” (and not with phosphoric acid)

How much carbon should I use?

This varies. Too many variables exist to provide a recommended baseline quantity of carbon. I've read anywhere from 36 ounces per 55 gallons (which is a LOT of carbon) to 7 tablespoons per 20-40 gallons. I would start with a smaller quantity and monitor water clarity and smell to determine if it is enough. Keep in mind, the less carbon that is used, the more frequently it will have to be replaced.

Carbon that is prepackaged in filter cartridges or media bags by filter manufacturers is generally sized to filter the tank size supported by the filter.

How do I prepare carbon for use?

It is advantageous to soak carbon in a salt solution (seawater strength) to ensure that if phosphate leaching occurs, it does not occur in your tank. Carbon should soak for 48 hours prior to use. However, this is not always practical. By selecting quality carbon phosphate concerns are primarily mitigated.

It is essential that carbon be thoroughly washed prior to use. It should be washed until the water runs clear and no dust particles remain.

How often should carbon be replaced?

While carbon manufacturers may advertise a suggested monthly replacement schedule I think this is somewhat optimistic. If you intend to use carbon then be prepared for the expense of replacing it every two weeks. You also have the option of including carbon in a media bag for a few days each week. Drop the bag into an HOB filter, allow it to run for about 12 hours, and then remove it, allowing it to dry out. Perform this task a few times a week and you should gain the benefits of activated carbon while significantly lessening the expense. But keep in mind, carbon does not stop working when you remove it from the tank. It will capture the same stuff from the air. So be sure to store it in an airtight container or ziplock bag.

Are there alternatives to carbon?

Yes, products such as SeaChem Purigen and Boyd's Chemi-Pure. Purigen is a synthetic adsorbent that does not remove trace elements but is effective in the removal of dissolved organics. In addition, Purigen is easily regenerated by soaking in household bleach. However, this regeneration process is not without risks so it must be researched prior to undertaking.

Boyd's Chemi-Pure is a carbon based resin that serves much the same function as carbon. But I'm sorry, I don't get the entire “Ion Antagonism” concept with “happy” and “unhappy” ions.


I remain of the opinion that carbon use, especially in a fish only tank, can provide benefit. However, this benefit is somewhat mitigated by the expense. So it is up to the individual aquarist to decide if the benefits are worth the expense.

One point of importance, never subsitute carbon filtration for bio-filtration. In my opinion, the primary benefit of carbon filtration is its ability to enhance bio-filtration, but if you have limited media space, bio-media is much more important.

Finally, carbon (or any other form of chemical filtration) is not a substitute for water changes. It (they) may be used as an aid to provide as pristine water conditions as possible, but they do not eliminate, or reduce, water change requirements.