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A discussion of what Zeolites are and how they function

 By Jens Kallmeyer

[Editorial note:  The author wished to state that he does not advocate the use of any specific brand of Zeolites and/or additives. He is not connected with any company selling these products. Jens can be reached at]

Zeolite Filtration for marine aquaria became popular a few years ago. Since then, it has caused a lot of discussion among hobbyists. While some hobbyists consider it the "Holy Grail" of coral keeping and enthusiastically promote this technique, others condemn it with as much enthusiasm.

What is lacking in most of the Pro/Con discussion is an actual explanation of the principles that lie behind this technique. Unfortunately, little information that goes beyond, “Use as stated in instruction manual” is provided by the suppliers of Zeolite Filters! In the earlier days, when this technique was still brand-new, more than just a few aquaria suffered from severe coral die-off after switching to this technique. These unlucky events were not restricted exclusively to inexperienced beginners, but rather to those that have kept a marine aquariums for many years. There are many beautiful tanks with Zeolite filtration.  There is ample proof that the method does work very well, with massive improvements in keeping extremely colorful SPS corals. Why does it work in some tanks and not in others?

Zeolites have been well known in freshwater aquaria for a long time. Their ability to reduce nutrient concentrations quite rapidly tempted marine aquarists to try freshwater Zeolites in seawater tanks. In almost all cases these tests ended in a major catastrophe. So what is the difference between those "tank-killers" of the past and the new Zeolite filters?  It is definitely not the design of the filter itself, which is basically a large canister-filter with a high flow rate.

First, it is the kind of Zeolite used. Second, it's the way of using the Zeolite as part of a whole concept. Apparently, there are more things to consider than just buying a pack of Zeolite and tossing it into a canister-filter. Before going into the details, it is important to understand the basic principles behind this filter system.

Let’s start with the basics and ask the question, "What actually is a Zeolite?"  Zeolites are minerals, and consist mostly of silica and aluminium, plus other elements like Sodium, Potassium, Iron, and Manganese. The most interesting feature of Zeolites is their crystal structure. You can think of Zeolites as a sponge with holes of different sizes.  The small holes are the size of single molecules, about one billionth of a meter. The size of these holes depends on the chemical composition of the mineral. There are hundreds of different Zeolites known in nature, and also many different synthetic Zeolites.

What makes these minerals so special is their ability to absorb specific compounds. Depending on their composition, they preferentially absorb different compounds. This means that they are more likely to absorb one compound more than the other if both are available. For many industrially-used Zeolites, their absorption preferences have been determined. However, thinking of Zeolites just as "absorbers" is a bit oversimplified. They are actually ion-exchangers. The holes to which a compound binds are not empty, but rather filled with sodium or potassium. As soon as a preferred compound is available, another compound, usually--sodium or potassium, is given off, and the compound  will be absorbed. This reaction, the replacement of one ion for another ion, is called ion-exchange.

The diagram on the left represents fresh Zeolite. Sodium (Na) and Potassium (K) ions are ready to be exchanged with other positively charged ions. The diagram on the right represents the release of Sodium and Potassium in exchange for Ammonium ions.

This "ion-exchanger effect" itself is of limited importance for the use of Zeolites in filtration of marine aquaria (there are some other tricks that will be explained later). However, it is of great importance in industrial applications. It is possible to produce synthetic Zeolites with very specific ion-exchange capabilities that can be "pre-loaded" with specific ions which will then be exchanged against other ions. In marine aquaria, the liberation of sodium or potassium is most probably negligible, as they are major constituents of seawater anyways.  

Why did so many earlier trials with Zeolites in seawater end unsuccessfully, even though they worked perfectly in freshwater? The answer is easy: The Zeolites commonly used in freshwater adsorb ammonium, which is a desirable function in fresh, AND seawater. However, Zeolites do prefer calcium. Now, you can imagine what happens in seawater!  There is usually little to no calcium in normal freshwater, so the Zeolites absorb ammonium. Because there is a lot of calcium in seawater and because these specific Zeolites prefer calcium, the calcium values drop instantly, with sometimes catastrophic results. In the earlier days, when marine aquarists experimented with freshwater Zeolites, they ended up with very low calcium concentrations of less than 200 mg/l.

The Zeolites now used for seawater preferentially absorb ammonium, but this is just one half of the story. The other half is where the biology comes in. As already mentioned, Zeolites have a very porous structure. Under the microscope, they look almost like a sponge. The larger holes are MUCH bigger than the smaller ones, about a thousand times bigger. This porous structure creates a large surface area for bacteria to settle. As the ammonium is adsorbed by the crystal structure, the bacteria living on the Zeolite get their food delivered to their doorstep. To enhance the filtration capacity, a carbon source is added, in most cases not directly into the filter, but into the aquarium. In most cases, aquaria are carbon limited.

Therefore, by adding a carbon source, all bacteria in the tank receive some additional food. But as those bacteria that sit on the Zeolite get the ammonia much easier and in much greater quantity than others in the tank, they can make much better "profit" from the carbon addition. To speed up the starting of these filters, some companies offer bacterial starter cultures, although their usefulness is debatable.   

Now that we have discussed the basic principles of what happens on the Zeolite, we should take a look inside each grain. Due to the porous structure of the material and the bacterial films coating the surface of each grain, oxygen will rapidly be depleted inside the Zeolite. At the very surface, where oxygen is still available, the ammonium is either oxidized by autotrophic bacteria to nitrate, and then immediately reduced to nitrogen gas by heterotrophic bacteria, or the ammonium may be reduced to nitrogen directly.

The latter process (anaerobic ammonium oxidation, anamox) is known to occur in sewage treatment plants, and has only recently been identified in nature for the first time. Proof of whether or not this process is occurring in a Zeolite filter would be highly difficult, if not impossible. This shouldn't bother the users of these filters; the net reaction is the same:  ammonium is removed from the water and transferred to nitrogen gas, which goes off into the atmosphere. All these reactions can only take place when a carbon source is added. Without a carbon source, the filter would first only absorb ammonium and nothing else. It would be a simple ion-exchanger. After a few days to weeks, chemoautotrophic bacteria would settle on the ammonium-loaded Zeolite and oxidize it to nitrate, as indicated in the following reaction:

NH4+ + O2 + 3OH-= NO32- + 2 H2O

Therefore, such a filter would actually produce nitrate, and its use for aquaria would be rather limited!   When I said previously that the ion-exchange process itself is not of much importance in the marine aquarium I may have been a bit too simplistic. Over the last year or so, many people actually have used this effect in a very elegant way. In new tanks, a little powerhead with a cartridge full of Zeolite can have a very positive effect on the stability of the tank. As ammonium is oxidized to nitrite, a reduction of ammonium levels reduces the nitrite peak in the start-up phase considerably. As the excess ammonium is taken out of the system before it is oxidized and starts causing problems, the Zeolite filter acts as a buffer which stabilizes the nitrogen cycle in the tank. In such a case, the Zeolites have to be changed every few days and no carbon source is added.      

In the “normal” Zeolite filters with the addition of a carbon source, these filters can run for extended periods of time because ammonium is constantly removed from the minerals by the combined efforts of autotrophic and heterotrophic bacteria. After a while, the Zeolite is exhausted and needs to be replaced. If the bacteria remove the ammonium from the minerals why doesn't the filter run forever?  First of all, the bacterial films will slowly clog up the pores, thereby reducing the adsorbing capacity, secondly other ions will also be adsorbed onto the Zeolite. As the bacteria do not remove these ions (at least not preferentially), they will slowly become enriched and therefore reduce the number of places available for adsorbing ammonium. In industrial applications Zeolites are often recycled. This is definitely not an option for the home aquarist, as it involves the use of highly aggressive liquids. If you try to do this in your kitchen you endanger your family's health and that of other people!  Saving money this way is just downright foolish. If you decide to set up a marine tank, make sure you can afford to maintain in properly.

There is also another method of nutrient removal working in Zeolite filters. As the bacterial biofilms build up, a lot of nitrogen compounds are actually fixed in the biomass and thereby removed from the water. When the biofilms become too thick and rip off, the skimmer may catch them and thereby remove these biologically fixed nutrients from the water.

The diagram on the top represents bacteria taking advantage of the locally high concentration of Ammonium trapped in the Zeolite and breaking it down to Nitrogen gas. The diagram on the bottom represents well fed, nutrient rich bacteria that have reached the end of their lives drifting from the Zeolites, destined to be skimmed out of the tank.

Since these filters entered the market, they have been proven to keep nutrient concentrations at levels usually found in the central Pacific, one of the most nutrient depleted waters on Earth. Extremely colourful SPS imports from Fiji, Tonga,  etc., that often went brown rather soon, now keep their amazing colours. For many users, this system provided a breakthrough in SPS-keeping. Even some long-time SPS enthusiasts with amazingly beautiful tanks running on different filtration systems seem to get more colourful corals after changing to Zeolite filtration.

The major advantage of these filters is also their major danger: They remove ammonium very rapidly and extremely efficiently. Although ammonium concentrations are never high (at least they should not be), it is a very important component in the nitrogen cycle. By removing it almost completely, all other processes will be influenced as well. When these filters entered the market, some people underestimated their effects on the overall chemistry of the tank. In rather old tanks with higher nutrient concentrations, the corals were well adapted to these conditions and the sudden drop caused massive die-off of colonies that had been growing well over decades. The corals starved and became susceptible to parasites and diseases. If the corals did not starve to death, the parasites killed them. It seems that tanks that have started with Zeolite filtration run very well and without major problems, while old established tanks are much more sensitive to it.

When switching from a different system to Zeolite filtration, some reports mention times of up to one year before the effects of the new system came fully apparent. Don’t forget that Zeolites are the most effective ammonium absorbers available. Keep a close watch on your corals, and measure the nutrient levels at least once a day over the first week. If  the corals react too drastically to the changes in water quality the amount of Zeolite needs to be reduced.

If you are tempted to try your luck with such a system, go ahead. It is a good way to an amazing aquarium. It is not much easier, cheaper, or better than several other methods that are available on the market--it is just different. It will not liberate you from proper and regular maintenance of your tank, and there is no guarantee that your tank will look like the one that made you consider trying this method!   Whatever you do, be conscientious and patient. No system can cope with the lack of knowledge, laziness, and impatience of the owner. Happy Reefkeeping!

WWM on Chemical Filtrants Marine in Filtration  

Related FAQs:  Zeolites, Marine Maintenance, Nitrates 2, Nitrates 3, Nitrates 4, Nitrates 5, Nitrates 6, NitritesAmmonia, Phosphate, Silicates, Biological Filtration, Fluidized Beds, Denitrification/Denitrifiers, Wet-Dry Filters, R.O./Distilled/Treated Water

Related Articles: Marine Chemical Filtrants,


Cellular Zeolite - 12/21/2005
Do you know anything about Cellular Zeolite? I purchased some for my mother to take to help her fight cancer. I've heard the claims made by a number of manufacturers (e.g. Zeolite "captures toxins" and removes them from the body). At the same time, because Zeolite contains silicates, I've heard there is a potential that it could actually cause health problems in some individuals.  
<Christine, I'm very sorry to hear of your troubles. I guess our site came up on a Zeolite search, but we aren't qualified to answer such questions. I would speak with a health care professional or perhaps even a poison control center. Maybe try searching the FDA for this, should have some info. as to possible adverse reactions. Best of luck. - Josh>


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