Zeolite for Ammonia Removal

Clinoptlolite is a natural occurring zeolite, only one of many types of zeolites found on earth. It has the ability to, inter alia, adsorb ammonia molecules and for that reason used by fish keepers to limit ammonia concentration in the water. It can be recharged with a brine solution, washed and used again. They are an essential part of aquarium filter maintenance and very valuable to keep your home aquarium safe for the fish inhabitants.

However, lately it is being used in sand filter housings as a replacement for ordinary silica sand in of high pressure filter systems. It is supposed to perform as as physical filter, ammonia adsorbent and biological filter, but it is our view its use may be a bit misguided.  

We tested a locally mined product. It is specified as having a cationic exchange capacity of 16 mg/kg. This is the total exchange capacity, reserved for all cations including ammonia, heavy metals, calcium and the like.

 Physical Filter:

As physical filter with particle size of 1-5 mm nominal, it would work much the same as conventional sand. It will also clog as a conventional filter, and it will require the same amount of maintenance. The loss in pressure (we guess) will be much the same.

Ammonia Adsorbent:

In repeated tests we recorded an absorbency for ammonia of not more that 9.3 mg NH3-N per kilogram for the zeolite. That it works as intended is not disputed, but the concern is the misunderstanding it might create with consumers about the real capacity.

One 25kg bag will adsorb only 230 mg ammonia. Full stop.

To get an idea of the magnitude of such a capacity, we just need to calculate that 6 koi at average 37 cm size and build (790-850g) will represent a body mass of 5 kg total. If they are fed the minimum of 2% of body weight of a 30% protein food (maintenance), that is a mere 100g daily diet. That 100g food will generate 2400mg of ammonia. The equivalent of almost 10 bags of zeolite.

No matter how wrong our data may be and/or how underfed the fish, the adsorbent capacity alone will have an insignificant benefit to the fish.

Another way to look it: If the six fish were in a 10,000 liter pond, fed 100g feed, no filtration but with a bag of zeolite, the ammonia concentration will already be 0,2 mg/liter after that day.

SO for koi ponds the fact that it adsorbs ammonia is not significant. Use it just as a physical filter and a biological media.


Biological Filtration:

Biological filtration will be much the same as with sand. Although the zeolite is specified with a huge 42m2/g surface area, it has significance only to molecules adsorbing, not at to the living space of nitrifying filter bacteria. One has to assume that only the outer surface of the zeolite particle can be populated. Hence the similar biological filtration ability.

It should be noted that clinoptlolite has a relative high pH of 8-9. The pH should be checked at first. Later the biological activity of several processes will normalize the pH.


The effect of salt:

It is a concern of koi keepers that the standard salt treatment of 0.3-0.5% salt might release the adsorbed ammonia and therefore pose a potential threat. We did not observe any release of ammonia except with high concentrations of brine.

This is still a subject of further investigation. Increase in salinity affected the chemical tests we were using. We will report as soon as we can determine the concentration môre accurately at which ammonia starts being release. That will calm the nerves.



The value of zeolite should not be underestimated. It is widely used as an adsorbent and find a wide diverse rage of applications. However in a environment where ammonia is produced at a high rate like in fish culture, it is doubted if it can find practical use other as a filter media.


Servaas de Kock

Additional information

Zeolites are microporous, aluminosilicateminerals commonly used as commercial adsorbents. They are a clay type mineral that was formed as a result of volcanic activity in ancient times.

Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. Adsorption is a surface-based process while absorption involves the whole volume of the material.

Microporous material is material containing pores with diameters less than 2 nm like zeolites. This is too small for nitrifying bacteria.

Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen. They also have some other ions in their crystal latticework giving its particular physical properties adsoption and high cationic exchange capacity. They are a major component of kaolin and other clay minerals.