A protein skimmer can be an indispensable
piece of equipment for marine and reef aquariums.
Since it is not a simple plug and run filter,
an understanding of the protein skimmer is
required in order to properly adjust and maintain
this mechanical device.
The main purpose of skimming is to remove
organic matter from the water before it decays.
Organic matter that has not been removed will
break down to ammonia, nitrite, and nitrates.
Organic matter, also referred to as dissolved
organic carbon (DOC), are carbon based compounds
that occur as part of the metabolical processes
and by oxygen consuming bacteria (i.e. the
beneficial bacteria). Some algae are also
known to create some DOC’s as part of
their normal biological function.
DOC’s are carbohydrates, fats, amino
acids, enzymes, hormones, phenolic compounds,
etc. Phenolic compounds appear as intermediate
compounds of chemical reactions and are increasingly
toxic with higher salinity and declining water
quality.
An interesting aspect is that skimming will
reduce pheromones. This hormone is released
by the fish as a form of chemical communication.
Pheromones influence the fishes’ behavior
such as reproduction, migration, schooling,
feeding, predator-prey and other interactions.
Pheromones in higher concentrations will suppress
the immune system of the fish making them
more vulnerable towards disease and parasites.
As it is the same with high DOC levels, a
protein skimmer will lower them to healthier
levels.
High DOC levels will inhibit the beneficial
bacteria colonies thus vastly lowering the
water quality. Rising levels can promote algae
growth, parasites flukes and protozoa thrive
in high DOC’s. An increasing biological
oxygen demand (BOD) can be noted since the
breakdown of this waste requires oxygen (Increasing
BOD levels are an indication of declining
water quality).
Mechanical filtration removes particles (fish
food, feces, mucus, etc.) from the water column
before it can decompose. The remaining particles
which are usually dissolved can be removed
with a protein skimmer. In short: protein
skimmers remove particles which a mechanical
filter does not.
In a nutshell, the working principle is that
waste particles are driven out of the water
by injected air bubbles.
More technically, the molecules to be removed
have two ends, one is attracted to water (hydrophilic)
the other is not (hydrophobic). These molecules
are called surfactants as they are attracted
to the water surface because of the air to
water interface. Rising air bubbles have the
same effect, the molecules will attach themselves
and thus being transported out of the water
and captured in the collection cup. This organic
waste can be observed as a ‘slimy’
water surface in some problem ridden aquariums.
The protein skimmer has 3 variables, flow
rate, bubble size, and contact time, which
can be summarized as follows:
The larger the bubble and the faster the flow
rate, the less contact time is given and the
contact time essentially determines the effectiveness
of the protein skimmer.
Despite the placement options which include
in-tank, hang-on, or sump units, a protein
skimmer differs in the way air and water are
mixed to allow the most efficient contact
time with the waste to be filtered.
The most basic skimmer is equipped with a
simple airstone connected to an air pump which
drives the bubbles upwards into the collection
cup (co-current). A more efficient version
would include a water pump that creates a
fast moving stream into which the air-bubbles
are injected. While advanced units first inject
a highly aerated water stream downwards (counter-current)
to allow a longer contact time.
The two different types of skimmers are the
air driven “columnar” style (systems
using an air pump) which are less expensive
but bigger in size, and the water driven (using
a water pump) “venturi” style
skimmer, more expensive but considerably smaller.
