Absorbing Medias

Adsorbing and Absorbing Medias 

by Tallulah

In this article we’re going to talk about three chemical filtration medias that are widely used by aquarium hobbyists; Zeo-Carbon, Activated Carbon and Zeolites.  We will not only be looking at how these can be used to our advantage but also the dangers of using them.

Absorption and adsorption are two different processes, although fairly similar.  When substances are adsorbed their atoms, ions or molecules bond to the surface or pores of the adsorbing media, think of this like iron filings being drawn to a magnet.  Whereas with absorption the entire media is used to hold the substance, a good example of this is water being soaked up by a sponge.


 Zeo-carbon, as the name suggests, is a mixture of zeolites and carbon.  Ammonia doesn’t bond well with activated carbon and therefore will make very little difference to ammonia levels, so the addition of zeolites ensures the ammonia adsorption is to the highest degree.  Having said this carbon is the best and most effective way of removing ammonia fumes.  However zeo-carbon is not particularly cost affective.  This is because carbon has a maximum life span of three weeks in comparison to the six weeks zeolites are active.  So theoretically you are paying for zeolites but only getting half of their use.  Zeo-Carbon comes in two ways, as loose chips with the materials mixed up together or as a sponge; commonly the sponge is activated carbon with zeolites impregnated in the centre. Both carbon and zeolites have their pros and cons which we’ll look at individually in the following paragraphs.

Activated Carbon

 Activated Carbon is a charcoal based substance that can adsorb certain chemicals and absorb organic matter.  There are two processes that can produce activated carbon, Physical Reactivation or Chemical Activation.  Carbon is formed from carbonaceous materials such as wood, coal and nut shells to name but a few.  Only carbon made from wood or coal are made for use in fish tanks.

Physical Reactivation

 The first stage in this process is the material, such as wood or coal, being put through a process called Pyrolysis.  Pyrolysis basically means the material is exposed to high temperatures in an oxygen free environment containing argon or nitrogen gases, creating carbon as a result.  Oxidizing is the next stage, this involves the carbonized material being exposed to steam or oxygen at high temperatures.  The oxidizing process causes many tiny pores to open as a result, producing activated carbon.

Chemical Activation

 Before the material is carbonized a strong chemical such as acid or salt is embedded in to it.  After this the material is carbonised by being exposed to lower temperatures in comparison to Physical Reactivation.  With Chemical Activation the carbon forming and activating happens simultaneously.  This is the preferred method of making activated carbon as it is the quickest and cheapest process.

What You Need to Know 

It’s vitally important to replace carbon, or any media containing carbon, every two to three weeks.  If carbon is left neglected in the tank or aquarium over a long period of time it can dissolve in to the water, this may very well be one of the causes of lateral line erosion.  Carbon will also remove certain vital minerals and can possibly shorten the fish’s life span as a result.

Absorbing medias

The red line shows where the lateral line is.  This is a sense organ that all fish have to detect movement and vibration.

One thing carbon does help with is breaking down organic matter.  Heterotrophic bacteria forms in all aquariums but they particularly like carbon to build their colonies around.  It’s this bacteria that uses waste as a food source and produces ammonia.  Heterotrophic bacteria is both anaerobic and aerobic, this means it can live in both heavily oxygenated water as well as environments with low oxygen.  The trouble with Heterotrophic bacteria is it can compete with Autotrophic (nitrifying) bacteria for oxygen and surface space.  Heterotrophic bacteria can multiply every 30 minutes whereas Autotrophic bacteria can take 12 to 24 hours.  Adding carbon can actually strengthen biological filtration as a result of providing an ideal surface area for Heterotrophic bacteria, which in turn leaves more space for nitrifying bacteria elsewhere in your tank or aquarium.  However I would still not recommend long term use of carbon, although it may be worth adding activated carbon on a temporary basis if you’re having trouble starting a cycle.  This would give the nitrifying bacteria more of a chance to mutate and build up a stronger colony, especially in an over stocked tank.

The most recommended way to use activated carbon is in emergencies.  It’s always good to keep some carbon sponges or chips in your store cupboard.  If dangerous medication is present in the water or someone uses hair spray near the tank for example, using carbon combined with water changes will ensure all the toxins are removed.  Keep the carbon in the tank for a minimum of 48 hours.

Below is a table of the many toxins activated carbon will remove or reduce.

Absorbing medias

*Please check the label on the treatment package to fully ensure carbon will assist in removing medication.

**These are treatments that this site regularly recommends, please make sure carbon is removed.

*** Caffeine is in black tea, green tea and white tea.


Zeolites, sometimes referred to as molecular sieves, are micro porous minerals that can both adsorb and absorb selected compounds.  There are two types of zeolite, natural and synthetic.  The formation of natural zeolites began millions of years ago with massive volcanic eruptions.  Eventually the ash clouds from the volcanos came to rest on the earth’s surface.  When the ash was exposed to water by landing in lakes and other water sources or the bed of the ash was penetrated by water; the ash reacted to the alkalinity in the water and zeolites were created as the result.

Synthetic zeolites are the most widely used type.  This is because most natural zeolites are contaminated by other minerals and have less adsorption ability, for example natural zeolites do not adsorb Sulphate Ions.   Synthetic zeolites are also superior to the natural ones in the fact they have a larger number of pores.  There are many ways synthetic zeolites are formed but the most used process is done by mixing up a gel using aluminium, and silica with steam.  This is then left to mature and later warmed up to around 194f or 90c. Another fairly common method involves the use of a type of clay called kaolin, this is heated in a furnace until it begins to melt, then it is cooled and ground down in to a powder form. The powder is then mixed with sodium and water, after this the mixture is left to develop and later heated.

Absorbing medias

What You Need to Know

It’s extremely important that you remove zeolites from your tank or aquarium if you intend on performing a salt treatment.  Zeolites attract salt and will leach out any ammonia that they have adsorbed, resulting in the fish being poisoned.  The good thing here is that the fact zeolites release the ammonia they are holding means you can recharge them.  This is done by simply placing the bag of zeolites in to a gallon of water with one pound of dissolved salt.

Using zeolites in a new set up will slow down the formation of nitrifying bacteria.  However it won’t stop the tank cycling in the long run as the bacteria will house it’s self in the micro pores of the zeolites.  If you have a cycle free tank remember to change the zeolites regularly and test for nitrite every five days.  I recommend leaving the zeolites in place if you discover nitrite, do a 20% water change daily, adding enough Seachem Prime or Amquel Plus to treat the entire tank and allow the cycle to complete.  If you do not wish to keep the zeolites in your tank on a long term basis, slowly turn the power of pump or filter down over a six week period.  Make sure you have plenty of sponge or ceramic media for the nitrifying bacteria to transfer to.

Absorbing medias

One of the major draw backs of zeolites is the affect they have on the water chemistry.  Zeolites adsorb calcium and magnesium.  They often are used to soften water in many washing detergents and filters.  This is one of main reasons why I would not recommend zeolites on a long term basis.

Absorbing medias

Believe it or not zeolites can be useful in a cycled tank, in some circumstances.  A fine example of this is if you have a small child who is assisting with caring for the fish.  If the child is an inconsistent feeder the ammonia will spike frequently, this is where the zeolites are useful.  The zeolites will adsorb any excess ammonia that the biological filter cannot support, and in turn preventing the fish being poisoned by ammonia.

10 tablespoons of zeolites per 2.6 US gallons, per 10 litres or 2.2 imperial gallon, to reduce ammonia by 1.5 ppm.

Below is a table of toxins and minerals zeolites can remove:

Absorbing medias

*Zeolites adsorb some medications but not all, I recommend carbon to remove medication.

**Do not use with zeolites in the tank!

***Synthetic zeolites only.

****Black Tea, White Tea and Green Tea contain caffeine.

Red indicates evidence proven through experiment by article author.

Watch zeolites adsorbing neat 3% hydrogen peroxide http://youtu.be/7yfOVZDyuOU

Half an hour late the peroxide was still reacting with the zeolites.

Using Activated Carbon Chips or Zeolites with a Pond Pump

First of all I must stress; if you have a fully cycled tank do not remove the sponges or disturb the pumps.  The best thing to do is use a spare pump or buy one, it doesn’t have to be the size of your current pumps you can use a smaller one.  Below are the instructions for both types of pond pumps:

Pump with an Arm

First take a stocking:

Absorbing medias

Place a plain piece of sponge in the end of the stocking:

Absorbing medias

Sit the arm on top of the stocking, placing the sponge under it:

Fill the stocking with the desired amount of zeolites or activated carbon:

Absorbing medias

Tie the stocking or secure it with an elastic band:

Armless Pump

For this you don’t need aquarium sponge.

Take the stocking and wrap it around the pump.  Keep the zeolites or carbon on only one side of the pump and tie the stocking around the base covering the other side to stop waste accumulating:

Remember none of these are recommended for long term use but are ideal for emergencies.

Absorbing medias

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