A long-standing choice in saltwater biological filtration, wet/dry trickle
filters are now becoming a source of controversy with the ever-increasing
popularity of reef systems. Long thought to be fish, but not reef,
friendly, aquarists often think of wet/dry trickle filters as "nitrate
factories". Over time the biological media inside the filter becomes dirty,
which will cause a build up of unwanted, reef-harming nitrates within the
aquarium. However, with research and understanding, trickle filters can be
an asset to any aquarium.
Although there are many variations of wet/dry filters, they all operate
using the same basic concept: aerobic filtration. "Aerobic" implies that
the process can only take place in the present of oxygen. Therefore, the
more oxygen saturation this system has, the better it works.
Water is pumped from the aquarium, then by means of a drip/trickle plate or
rotating spray arm the water is dispensed or "trickled" down over and
through a biological material source contained in the wet/dry filter
chamber, but not before the water is pre-filtered by means of mechanical
filtration, which can be accomplished with the aid of a protein skimmer or
by placing a pre-filter material such as filter floss, a filter sponge, or
micron filter felt on top of the drip/trickle plate area. When the water
falls through the holes of the drip/trickle plate onto the bio-media, this
allows for aggressive oxygen saturation of the water. Remember, aerobic!
The clean filtered water is then deposited back into the aquarium either
directly, or first into a sump or some type of water containment area and
The Importance of a Pre-Filter Set Up
A pre-filter set up allows the tank water to be cleaned of excess debris,
particulates and other unwanted organics (DOCs or dissolved organic
compounds) before it passes through the bio-media in the wet/dry chamber.
By using this process it helps to prevent the bio-material from getting
clogged and dirty, as this is what can contribute to the build up of
nitrates in an aquarium. A double drip/trickle pre-filter plate design,
where one plate drips down onto the other, can also be incorporated. The
first plate holds the pre-filter material on it, and the second is just a
plain drip plate.
It is important to keep the pre-filter material changed. The purpose of the
pre-filter is to only collect unwanted junk in the water, and nothing else.
If the pre-filter material is not maintained regularly, this can decrease
the flow rate of the water that trickles into the wet/dry filter, in turn
decreasing the oxygen saturation level. This also allows the pre-filter
material to act as a contributor to the build up of unwanted nitrates in
The frequency that you change the pre-filter material depends on the animal
and feeding load you have on your system, but no less than a weekly change
is recommended, and it definitely should be changed when it begins to turn
About Choosing a Bio-Material
Just about anything can be used as a biological filteration material, but
ideally you want a type of media that has a large surface area for growing
bacteria, does not compress down, is saltwater friendly, and has good
water/oxygen exposure around and through it.
The top choice bio-media is usually in the form small spiked plastic balls,
or bio-balls, but there are many other types of bio-media one has to pick
from (see Choosing a Biological Filtration Material). Some commercially
sold wet/dry filt