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 firstname.lastname@example.org]
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
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
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
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
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
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
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
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
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
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
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!