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Using Activated Carbon in the Aquarium

Activated carbon is used quite commonly as part of the filtration system in aquaria, but there are many myths and misunderstandings about its use, and what it can or cannot do. Some of these myths are repeated so often that many believe them to be absolute truths!

Carbon will remove a range of substances from water, including:

  • Dissolved organic molecules - this is the primary reason to use carbon in most aquaria.

  • Medications, antibiotics and dyes - this can be useful for removing excess/residual medication, but also means that carbon should be removed when using medications, to avoid removal of the medication during the treatment phase.

  • Chlorine and chloramine (carbon blocks are often included in RO prefilters to avoid damage to the RO membrane by chlorine and chloramine).

  • Certain heavy metals - the amounts will depend on factors such as the solubility of the metal at the pH of the specific aquarium.

It is important to note that there are certain substances that carbon will not adsorb any significant amount of, which include:

  • Ammonia, nitrite and nitrate.

  • Phosphate

  • Inorganic salts (like sodium chloride)

Substances like nitrate and phosphate can be adsorbed by other specialised resins, and some commercial products are a blend of carbon and more specialised resins.

Note the use of the word "adsorb" with a "d". This refers to substances being bound onto the solid surface of the media, rather than being absorbed internally (in the way that a sponge absorbs water).

Apart from the substances that carbon will or won't adsorb, there are a number of other misunderstandings about carbon.

It is often said that exhausted carbon will leach adsorbed substances back into the aquarium. This is unlikely under 'normal' conditions. Specific chemical conditions are required to remove many substances once adsorbed, such as extreme pH values outside the normal range for aquaria. All that will happen if you leave carbon too long without changing it, is that it will no longer be effective for its intended purpose, and will instead become coated in a biofilm and become part of the bio-filtration. However, if this is then removed, part of the biofiltration capacity has been lost, so it makes sense to either change carbon regularly so that it continues to perform its intended task, or if you choose not to use it, replace it with extra biomedia.

Another concern about using carbon is the possibility of removing desirable 'trace elements' from the aquarium water. Although this is possible, in practice the desirable substances concerned (such as iron in a planted aquarium) are unlikely to be removed to any significant degree under normal aquarium conditions.

How much should I use, and for how long?

It's almost impossible to recommend a specific amount of carbon, or a specific length of time that it will last in a particular aquarium. This is because it will depend on many factors, and every aquarium is different. The biological load (stocking level) in the aquarium will have a major influence, but the quality of carbon itself (and therefore its adsorption capacity) can also be very variable.

Some manufacturers provide a rough guide as to how long their carbon should last. This can vary from one to four weeks for most 'carbon pads' and 'carbon sponges', to a few weeks for most loose granular carbons - or as long as three months may be claimed for some high grade carbons. Of course this all depends on the amount used in relation to the aquarium size and stocking level.

For carbon to work effectively, the water should be mechanically prefiltered. The carbon should have a good flow of water through it, so when using granular carbon, a thin layer is more effective than a thick bundle.

So do I really need to use carbon?

In most aquariums, if you carry out regular water changes every week and maintain the aquarium properly, there shouldn't be a definite need for carbon. However, given some of its benefits, it certainly won't hurt to use it, as the benefits far outweigh the potential concerns (many of which are exaggerations or myths anyway). Many people adopt the sensible middle ground and use carbon as and when needed, such as removing residual medication or discolouration from bogwood from the water.

However, others feel that carbon is a good 'safety net'. If replaced regularly, it would take up chlorine compounds if there is any residual not eliminated by the water conditioner, it can remove all sorts of potentially nasty organics that could be in tap water at low levels; removal of more general organic wastes will promote more efficient biofiltration, promote higher oxygen levels and help to avoid sinking pH. It should help to remove pheromones given off by fish that can inhibit their growth. The use of fresh carbon tends to give water that extra crystal-clear 'brightness' and can give older established aquariums a water quality boost. It's worth noting however that many of these things can also be acheived largely by doing regular water changes!

If your filter system comes with carbon media, you may as well use it for the recommended time. Whether you replace it with fresh carbon on a regular basis is a matter of personal choice, and the cost of replacing something you don't have to use may be a factor. Remember that biological filtration is the most important part of the filter's function, so if you decide not to use carbon, then replace it with extra biomedia instead (either specialist biomedia or extra foam/sponge where relevant).

A final myth about carbon is that you can recharge it yourself in a domestic oven, allowing it to be reused. To purge carbon of impurities requires temperatures of more than 1000oC (over 1800oF) at a specific pressure - in other words, don't waste your time or your fuel bill trying to do it in a domestic oven!




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