Zeolite.
The term zeolite (translated from Greek as “boiling stone”) encompasses an entire family of minerals – water-containing aluminosilicates with potassium, sodium, calcium, and magnesium cations. There are natural and artificially synthesized zeolites (permutites) that are widely used in water purification devices as adsorbents, ion exchangers, and molecular sieves. What exactly is a zeolite? To put it simply, it is a natural mineral belonging to the aluminosilicate group.
It possesses a characteristic structure abounding with cavities occupied by large cations and water molecules. Both of these are characterized by significant mobility, which enables ion exchange (large ions are sometimes called “exchangeable”) and dehydration (release of water). The framework structure of zeolite is built of vertex-connected tetrahedra. Oxygen atoms occupy the angular positions, while the centers are filled with so-called small atoms, or T-atoms. In most natural zeolites, the T-atoms are aluminum (Al) and silicon (Si). Single- or double-charged cations Na+, Ca2+, K+, Mg2+, Ba2+ play the role of large ions in the cavities.
A large number of minerals belong to the zeolite group. Zeolites are produced for agriculture, water and gas purification, as livestock feed additives, cat litter fillers, and sprinkling agents for animal housing areas. Many companies produce zeolite as an ammonium absorbent for aquarium filters. Practically all of them are suitable for an aquarium, as long as it is not a sodium zeolite! If the ion-exchange sites of the zeolite are occupied mainly by potassium [K] (potassium zeolite), such a zeolite is suitable for growing plants. If the exchange sites are occupied by sodium Na (sodium zeolite), then such a zeolite must not be used! Agricultural zeolites are always potassium-based. Sodium zeolite is used for liquid and gas purification.
There are 48 species of zeolite found in nature and over 150 artificially synthesized species. Importantly, the properties of natural zeolite do not depend on granule size. Its structure does not change under pressure, high temperatures, chemical exposure, or time. Unlike artificial zeolites, natural zeolite does not disintegrate in a weakly acidic environment (such as in a substrate with peat). They all differ in composition and have different names and properties. Therefore, when referring to a specific type of zeolite, it is more appropriate to use its specific name rather than the general group definition.
Only 8 of the known zeolites have major significance (mostly industrial for now): analcime, chabazite, clinoptilolite, erionite, ferrierite, laumontite, mordenite, and phillipsite. Their most typical field of application is water purification. The ion-exchange capacity of zeolites ensures the extraction of various metal cations, as well as ammonium ions, from the water being purified. In addition, zeolites have high dirt-holding capacity due to the large inter-granular space during backfilling. This ensures water purification from suspended coarse and colloidal particles as well.
Which of the zeolites do we deal with in aquarium hobby? Most zeolites found on sale are mined at the Sokirnitsa deposit (Ukraine). Beyond the Urals and in Siberia, zeolites from the Kholin deposit (Buryatia) are more common. Household water filters have even been manufactured based on them. In southern regions, zeolites from the Nor-Kokhb deposit (Armenia) are sometimes found. At the same time, clinoptilolite Clinoptilolite makes up the lion’s share of zeolites from all listed deposits. Clinoptilolites are a particularly valuable variety of zeolites. They are widespread in nature and find application in agriculture.
Zeolite is a porous natural mineral containing up to 70% clinoptilolite, with impurities such as montmorillonite, quartz, feldspar, opal, volcanic glass, etc., with a silica-to-alumina ratio of 3.5 to 10.5, and containing an average of 60% silicon dioxide. In clinoptilolite, the diameter of the entrance windows into the cavities is 0.4 nm. And it is this mineral that determines the properties of the mixture as a whole. Clinoptilolite is primarily a cation exchanger. By absorbing certain cations, it replaces them with others.
Depending on the type of clinoptilolite, the exchangeable ions can be Na+ or K+. Therefore, when using this mineral, the content of Na+ and K+ in the water increases slightly, or both ions at once. However, this process usually cannot be accounted for because the initial composition of each batch of zeolite and the specific exchange ions it possesses are unknown, as the nature of exchange ions varies greatly from batch to batch.
The use of zeolite in aquarium keeping is primarily associated with its ability to cleanse water of ammonia and ammonium. The mobility of cations and their capacity for ion exchange determine the high sorption properties of zeolites. In simplified terms, this can be described as: zeolite “takes away ammonia” and “exchanges it for salt”. However, as practice has shown, our understanding of zeolite’s ability to combat ammonia is somewhat exaggerated. In reality, not all types of zeolites are suitable for this. A common structural feature for all minerals of the zeolite group is the presence of a three-dimensional crystal framework forming systems of cavities and channels, in which alkali, alkaline-earth cations, and water molecules are located. Cations and water molecules are weakly bound to the framework and can be partially or fully replaced via ion exchange and dehydration, reversibly and without destroying the framework itself.
Dehydrated zeolite is a microporous crystalline “sponge” with a pore volume of up to 50% of its framework volume. Depending on the zeolite type, such a “sponge” has a pore diameter ranging from 0.3 to 1 nm. Each type of zeolite is capable of efficiently adsorbing molecules only within a specific size range. Micropore sizes determine the specific selectivity of zeolite properties as adsorbents, ion exchangers, and molecular sieves; thus, some zeolite types express certain properties while others express different ones. The ion-exchange properties of zeolites are also determined by the chemical affinity of ions with the crystal structure of the zeolite. Moreover, similar to molecular adsorption, there must be a match between the sizes of the entrance openings into the zeolite framework and the replacing ions. Ion exchange on zeolites allows the extraction of ions whose removal by other methods is sometimes extremely difficult.
First of all, it is worth noting the pronounced selectivity of clinoptilolite regarding NH4+ ions. Its static capacity for this element in practice amounted to 20 g/kg. This is slightly less than the values claimed by the manufacturer (usually declared as 25-30 g/kg). The fact is that technical documentation usually provides values measured in water containing no other cations. In aquarium water, however, there are plenty of them, plus quite high concentrations of soluble and insoluble organic compounds. This, in turn, affects the amount of trapped ammonium. Also, the amount of absorbed ammonium ions decreases at high concentrations of Ca2+ and/or Mg2+ in water, i.e., in hard water. Salinity also negatively affects the absorption of ammonium by clinoptilolite. Nevertheless, the absorption of large quantities of ammonium is the main property characterizing clinoptilolite.
Clinoptilolite can be most effectively used as a filler in a biological filter to reduce high concentrations of ammonia/ammonium in water. Typical application areas also include transport containers, overpopulated aquariums, retail and fish-holding racks in pet stores where high stocking densities are usually maintained, as well as breeding tanks with fry, where using zeolite helps reduce the frequency of water changes (especially for species sensitive to this procedure and water composition changes). Zeolite also serves well in preliminary water conditioning in regions where tap water initially has a high ammonium concentration. After passing through a simple zeolite filter, the water is ready to be poured into the aquarium.
The valuable properties of zeolite were previously used only in canister filters as an ammonium/ammonia absorber; now it is increasingly used in substrates for aquariums with 
plants. By modifying the composition of ion-exchange sites and charging sites with targeted nutrients, zeolites can become an excellent substrate for growing plants. Combined with slow-dissolving materials (such as synthetic and/or natural nutrient anions), these nutrient-enhanced zeoponics materials supply plant roots with additional vital nutrient cations and anions. Most importantly, these nutrients are provided slowly, according to the needs of the plant roots. Zeolite becomes saturated with nutrients that can only be extracted by plant roots. Therefore, over-fertilization of the water is eliminated.
How does it work? Generally, it is a process representing a combination of dissolution and ion-exchange reactions. The absorption of nutrients from the soil solution by plant roots triggers dissolution and ion-exchange reactions, releasing nutrients on demand. Afterward, the zeolite is recharged with additionally supplied nutrients. As a result, the zeoponics system enhances nutrient retention, reduces losses, and decreases the need for fertilization, creating a renewable and balanced delivery of nutrients to the root zone. Due to its properties, zeolite is one of the best substrate additives for planted aquariums and can serve as the primary base for the bottom layer. Due to its color (dark grey-green when wet), zeolite cannot serve as the primary substrate for top layers. Zeolite has major advantages over clay, sepiolite, peat, and vermiculite – it is physically stable and more convenient to use.
Zeolite can be purchased very inexpensively in gardening stores to obtain the same CEC as expensive, entirely anaerobic, and disposable clay additives. If calcined clays are unavailable to you, you can make the bottom layer using regular gravel mixed with 10-20% zeolite. Zeolite should be used moderately. They should occupy no more than 20% of the total volume. This is entirely sufficient to retain up to 80% of the NH4+ ammonium generated in the substrate. Attention should also be paid to grain size. Since plant roots receive nutrients through direct contact of root hairs with the substrate, a fraction of 2-5 mm should be chosen.
You can prepare nitrogen- and phosphorus-charged zeolite right at home in the kitchen. Before placing it in the aquarium, zeolite, like any other high-CEC material, can be charged with nutrients by immersion for 24 hours in concentrated solutions of NH4+, NO3, PO4, K, etc. This provides excellent plant growth immediately after aquarium startup and allows avoiding the addition of macronutrients other than potassium to the water for the first three months, which reduces algae growth in case of imbalances. It is recommended to charge zeolite with ammonium NH4+ by adding ~250 g of urea per 45 kg of substrate.
There is an opinion that zeolite performs very well as a substrate in planted aquariums. Both pure rock and its mixture with regular gravel can be used. Indeed, zeolite is capable of accumulating plant-essential substances such as ammonium, iron, manganese, potassium, etc. in its pores. And they will be located in the immediate vicinity of the roots. At the same time, clinoptilolite maintains these elements in the required ionic state, essentially “conserving” them in its pores and thus protecting them from oxidation. Such a substrate usually has a slightly acidic reaction. Zeolite substrate will successfully retain large amounts of ammonium. Its high cation-exchange capacity will also play to its advantage.
Clinoptilolite slightly softens and acidifies water while absorbing ammonium and iron. Therefore, it makes no sense to pour iron-containing fertilizers into an aquarium equipped with a zeolite filter. However, besides ammonium, this mineral is also capable of absorbing such important ions as copper, calcium, magnesium, iron, and manganese. Moreover, maximum absorption of Fe2+ and Mn2+ is observed in the pH range from 6.0 to 9.0. This makes the use of clinoptilolite in planted aquariums with dosed fertilizer supply rather questionable, since Fe2+ and Mn2+ are precisely the main trace elements there. However, they will be absorbed by the mineral and won’t reach the plants. At the same time, the presence of zeolite does not affect parameters such as nitrite, nitrate, and phosphate anion content. Clinoptilolite is not a good substrate for bacterial colonization. Bacteria colonize its surface and can process NH4+ sorbed in the zeolite pores, but the pores themselves remain inaccessible to them.
During practical application, another useful property of clinoptilolite was noticed. It is capable of removing pharmaceutical products from water. Naturally, zeolite handles copper-containing medications well. However, it turned out that other pharmaceutical agents are also within its power – methylene blue, trypaflavine, malachite green.
In conclusion, let’s summarize everything said and try to answer whether we actually need zeolite. It seems to me that every aquarist should decide this for themselves depending on the tasks they set. We see that zeolite will be a good assistant in decorative ponds with high fish density, in containers for fish transport, quarantining, and holding. It is also useful in breeding aquariums.
Also, this natural ion exchanger comes in handy when pharmaceutical products need to be removed from water after fish treatment. In regions with high ammonium concentrations in tap water, a zeolite filter will come to the aquarist’s rescue once again. However, as a biofilter medium, zeolite falls short of many modern highly porous materials due to its very small pore diameter. In a planted aquarium, zeolite will also be of little use, except perhaps as a substrate additive. So think, experiment, and decide.
Spent zeolite can be regenerated by soaking for 24 hours in a 10-15% table salt solution followed by rinsing in running water. However, one should bear in mind that using salt produces sodium zeolite, which will subsequently replace Ca++ and Mg++ with Na+, although if the zeolite is not meant for a planted aquarium, this is not a major issue.
Organic matter. Use of peat. Humic acids, Tannins.
Ion-exchange resins. Zeolites, Cationites, Anionites, Water deionization. 
