Zeolite and Ion-Exchange Media in Aquariums: Ammonium, Regeneration and Selective Resins
How zeolite exchanges ammonium, why salt regenerates it and why it fails in seawater, how nitrate- and phosphate-selective media work, and when ion-exchange media are worth using.
What zeolites are
Zeolites are microporous, crystalline aluminosilicate minerals. In their framework, some silicon (Si4+) is replaced by aluminium (Al3+), which leaves a net negative charge balanced by mobile cations, usually sodium or hydrogen. Those mobile cations can be swapped for others in the surrounding water, which gives zeolites their cation exchange capacity. Their best-known commercial use is in laundry detergents, where they remove calcium and magnesium from hard water in exchange for sodium. Clinoptilolite, the natural zeolite most associated with ammonium removal, has the formula (Na,K,Ca)2–3Al3(Al,Si)2Si13O36·12H2O and a strong exchange affinity for ammonium (NH4+).
How zeolite removes ammonium
UF/IFAS describes zeolites as ammonia-adsorbing clays that take up positively charged ions, including ammonium. In water, total ammonia exists as an equilibrium between NH3 and NH4+, with pH below 7 favouring the less toxic ammonium. A 2025 review in the journal Materials notes that clinoptilolite-rich zeolites remove ammonium because of their cation exchange capacity and their preference for ammonium over competing cations such as sodium and calcium that are common in freshwater. Capacity varies with the Si/Al ratio, pore volume, surface area and exchange capacity of the particular material, and at low pH ammonium uptake falls because hydrogen ions compete for the exchange sites.
Because zeolite is a cation exchanger, it does not take up nitrite or nitrate, which are anions; those are handled by biological filtration, water changes or anion-exchange media. Zeolite is also not perfectly selective. UF/IFAS notes that it may remove calcium and magnesium as well, and Wikipedia adds that its affinity for calcium can reduce its effectiveness in hard water while depleting calcium. UF/IFAS advises rinsing new zeolite under running tap water before use so that it does not cloud the aquarium.
Regeneration with salt
Exchange capacity is finite: once the sites are occupied, the medium stops removing ions. The household water softener shows how recharging works. According to NDSU and UNL Extension, a softener bed of natural zeolite or synthetic resin holds sodium loosely; hardness ions displace it during service, and when the bed is saturated it is flushed with a sodium chloride brine, whose sodium displaces the captured ions, which leave with the waste water. How often regeneration is needed depends on the hardness of the water, the volume treated and the capacity of the medium.
The same principle applies to ammonium. In a 2026 study in Scientific Reports, a sodium chloride solution was used to desorb ammonia from modified clinoptilolite granules, which retained moderate reusability over several cycles. The review in Materials states that natural and modified zeolites can be regenerated and reused many times, but also notes that long-term regeneration performance has rarely been studied. Sodium-treated zeolite tends to perform better because sodium exchanges more readily than potassium, magnesium or calcium.
Why zeolite does not work in saltwater
UF/IFAS states directly that ammonia-adsorbing clays do not work in seawater. This is consistent with the regeneration chemistry above: a concentrated sodium chloride solution is exactly what strips ammonium off zeolite, and seawater is itself a concentrated salt solution rich in sodium and other cations. In a marine aquarium the competing ions occupy the exchange sites, so zeolite cannot hold ammonium against them. Ammonia control in marine systems relies on biological filtration.
Selective resins: nitrate and phosphate
Synthetic ion-exchange resins come in two families: cation resins, which attract positive ions, and anion resins, which attract negative ones. Strongly acidic cation resins carry sulfonic acid groups and are used for softening; strongly basic anion resins carry quaternary amino groups. In general, ions with a higher charge bind more strongly than monovalent ions, and this selectivity decides how a resin behaves in mixed water.
Nitrate removal illustrates the problem. Anion resins in drinking-water treatment exchange chloride for nitrate. An EPA report on an ion-exchange plant gives the general selectivity order of a strong-base anion resin as sulfate > hydrogen arsenate > nitrate > nitrite > chloride > bicarbonate. Because sulfate is preferred, Penn State Extension warns that water high in sulfate hinders nitrate exchange, and once the resin is saturated it can release nitrate in place of sulfate, so treated water briefly contains more nitrate than the feed. The EPA report describes this as chromatographic peaking and notes that ion exchange becomes uneconomical when total dissolved solids exceed 500 mg/L and sulfate exceeds 150 mg/L. Nitrate-selective resins were developed for this situation and are named as a separate category in the EPA report. Spent nitrate resin is regenerated with a concentrated sodium chloride solution.
Phosphate can also be targeted. The review in Materials describes lanthanum-modified zeolites, whose lanthanum binds phosphate in stable complexes and raises selectivity for it. Phosphate remover resins used in aquaria are covered in a separate guide.
When ion-exchange media are useful
- Freshwater ammonium control in specific situations: a 2025 review in Molecules lists removal of ammonium from aquarium water, hatcheries and fish transport among the main uses of natural zeolites in aquaculture.
- Supplementary surface area: Wikipedia notes that porous zeolites also act as sites of bacterial colonisation in filters.
- Softening: cation exchange removes calcium and magnesium in exchange for sodium; UF/IFAS notes that aquarists may need to add some magnesium and calcium back after softening. Softener-treated water is discussed in its own guide.
- Nitrate or phosphate reduction in freshwater, where selective resins work against far fewer competing ions than in seawater.
In each case the medium has a finite capacity, its performance depends on the other ions present, and the result should be confirmed by testing ammonia, nitrate, phosphate or hardness rather than assumed.