Understanding Filtration

Most aquarists, I am sure, strive to keep their fish happy.  Happy as in healthy and thriving, not just bare surviving.  Personally, I consider temperature and good water quality as the two most important considerations in achieving this target.

As an amateur tropical fish hobbyist living near the tropics, I never ever had to suffer the bane of fluctuating or inappropriate temperatures.  My aquarium waters are kept at a constant 22 to 28 degrees Celsius year round.  My experiences of meddling with the water temperature are limited and therefore I am not in the position to write a lengthy article on it.  Interested readers can no doubt find detailed discussions elsewhere in the net.

Before plunging into the intricacies of water quality, I find it prudent to talk about the nitrogen cycle and its effects on the aquarium.  Generally speaking, nitrogen plays a very important role since it is the building block for many crucial molecules such as the amino and nuclei acids.  Nitrogen normally and primarily enters the organism cycles by being taken up by plants.  The nitrogen is transported into plant eating animals and then ultimately the carnivores.  The aquatic cycle performs in similar ways.

The problem with nitrogen compounds in organism diets is that they often produce unusable and toxic by products.  The organism either removes these or converts them to a harmless form.  A easily visualized method of removal is familiarly known as excretion.  Most if not all fishes have a highly developed system to perform this function.  This method of waste removal causes particulate pollution.  And when the waste breaks up and decompose within the aquatic system, chemical pollution also takes place.

Generally, the form the nitrogen waste exist in has significant effects on the way it is expelled from the body.  A single molecule is easily and most efficiently removed through the gill membranes of the fish as ammonia.  This discovery dates back to an experiment on gold fishes by HW Smith (1929).  It was found that the nitrogen excreted through the gills was ten times more than the other parts of the body combined.  However for this system to perform its job efficiently, water balance and the control of various other chemicals must be balanced.  For example, there is a tendency to exchange ammonium ions with sodium ions through the gill membranes.  However, increasing the ammonium concentration in the water inhibits the sodium uptake by the gill membranes.

In short, a fish must get rid of its nitrogenous wastes and this happens best under a given set of optimum conditions.  A freshwater life form should also take in body salts, both from its surrounding waters and from its food.  By confining fishes to a limited volume of water without sufficient water exchange, we can safely say that the fish is likely to suffer from a lack of body salts and also a build-up of nitrogenous waste in its body.

The above nightmare scenario can often be associated with insufficient or improper filtration.  Filtration, at its most basic interpretation, is the process whereby the aquarium water is purged of particulate, organic and chemical pollutants.  The process of removing particulate pollutants is called mechanical filtration.  Removal of chemical toxins and organic waste is termed biological filtration.  A third alternative is chemical filtration which basically involves the use of chemicals to perform the job of biological filtration.

Particulate pollutants exists in many forms such as excreted waste (as previously mentioned), uneaten fish food, cellulose materials from plants and fishes and many others.  Some of this particles remains in suspension and make the water appear dirty to the naked human eye.  The larger particles tend to accumulate at the bottom of the tank, and are occasionally disturbed by body movements of the fishes.

Besides being visually unpleasant, the accumulation of such materials provides a platform for parasites and other disesease causing pathogens to grow and at the same time weakens the ability of the fish to resist attacks by the same undesirable organisms.

The build-up of particulate contaminants may be slowed by water changes.  However there are not many aquarists who are willing to adopt a religious and frequent water change regime.  Besides, water changes, unless done very discretely invariably will stress the inhabitants to the inhabitants of the aquarium system.

Fortunately, there exists many devices developed for the aquarium hobby and they allowed the hobbyist to reduce the frequency of water changes.  Most of present day mechanical filtration removes the unwanted particles by continuously circulating the aquarium water through a physical screen that traps the particles.  The filter media varies widely from floss that trap only the largest particles (but is cheap and disposable) to diatomic filters that effectively "polishes" the waters.

In selecting the type of mechanical filter, one should always match the particle trapping ability to the requirements of the tank concerned.  A very fine filter medium will trap so many particles that it will clog up very quickly and requires frequent maintenance.  Another important consideration will be the flow rate of the filter itself.  For most aquariums the filter should circulate four to six times the volume of the aquarium.

A constant stream of metabolic by products such as proteins and amino acids is constantly disposed in the aquarium water by fishes.  These organic materials are dissolved in the aquarium waters and therefore known as dissolved organic carbon.  Laboratory tests have associated poor fish health and growth with increased concentrations of these organic pollutants.

Ammonia and nitrates are inorganic contaminants.  How ammonia comes into the aquatic system has already been discussed and I will not repeat myself.  The cause of nitrates will be discussed together with biological filtration in the next part of the article.  In any event, both organic and chemical pollution can be reduced by biological and chemical filtration, and of course, the ubiquitous water change.

Biological filtration is carried out by two types of bacteria, which can live on all surfaces in the system. In short, more surface area correlates to a larger bacteria colony. Nitrifying bacteria do NOT live free swimming in the water.  Somebody once asked me if using a bigger tank (bigger glass walls) will provide more surface for such bacteria.  The answer is yes, but the surface area of the glass is only a negligible portion compared to the volume of the water. The most common way to increase the surface area is done within the filtration system itself.  These methods commonly involve the use of gravel, sponges, ceramic rings or any material over which the nitrifying bacteria can spread.

Biological filtration is conducted in two stages, each stage being hosted by a type of bacteria.  The first type of bacteria (Nitrosomonas) absorbs ammonia and converts it to nitrite.  Another bacteria (Nitrobacter) absorbs the nitrite and converts them to nitrate.  Nitrate is absorbed by plants and algae as they grow.  Laboratory have proven that nitrates are much less toxic to fishes than nitrites (some 700 times, if I remember correctly).  The reverse is true for crustances which unfortunately suffer more as nitrates build up.

As the final point before I wrap up this discussion on biological filtration, I will like to caution fellow hobbyists that although nitrates are much less toxic than nitrites,  an unattended build-up of nitrates can still result in fatalities.  Unless a tank has an overabundance of plant material to "naturally" absorb these nitrates and prevent a build up, water changes are still necessary.  The best filtration system in the world only prolongs the period between necessary water changes.

Most chemical filters that are available in the market are centred around the use of activated carbon as the filter medium.  The choice candidate is high quality granular activated carbon.  This type of carbon was degassed in an oxygen oven at very high temperatures (temperatures way above those found in the home ovens).

Activated carbon removes pollutants via a process known as adsorption (note that adsorption differs from absorption).  As the contaminants come into contact with the surfaces of the carbon granules, they were attached.  When the surfaces of the carbon become saturated, it must be discarded and replaced with new carbons.

It is in the same line of reasoning that carbon filters should always be placed after mechanical filters, else the particles will quickly coat the surfaces of the carbon, rendering it useless.  Also, for a given amount of carbon, smaller granules perform their intended tasks better since more surface area is available.  However, breaking up big chunks of activated carbon will not improve the chemical adsorption abilities since the newly exposed surfaces are not yet activated.

We have come to the end of this lengthy discussion on filtration.  If you are game for more, drop in to Watery Games.

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