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The aquarium is a living environment and therefore some natural processes take place.

Fish excrete, plant leaves decay and uneaten food rots. All these processes contribute to

water contamination and because the aquarium is not affected by the constant cleaningeffects of currents, flow and rain present in the wild, the water can quickly become

turbid, harbour disease and poison the fish.

Very few aquaria are self sufficient in terms of filtration, that is can mimic without

intervention, the completeitrogen !ycle.The exceptions are extremely large aquariawith abundant vegetation, few moderately fed fish, and the presence of scavenger fish

and algae eaters. "owever, even those aquaria will still require water changes to replicate

part of the #ater !ycle. $n the days before the sophistication of modern filters, thecontrol of water quality was purely down to water changes and $%m sure we are all aware

of the inconvenience that this could cause. &o, to prevent the need for daily, or even more

frequent, water changes some form of filtration is required.

All filters carry out one or more of the following types of filtration' mechanical,

biological and chemical. $n addition, a filter can also be used to create currents andprovide aeration(oxygenation) through surface movement.

*echanical filtration removes particulate wastes from the water purifying it as it passes

through the filter media. +assing water through some form of sieve, a sponge forexample, essentially performs this. The filter media retains dirt and releases water. ature

performs this function as rain filters through strata in the earth. The finer the media, the

smaller the particles of waste which will be trapped and consequently the better themechanical removal of particles from the aquarium. The down side is, the finer the filter

material the more prone to clogging it will be. Virtually all mechanical filtering

inherently harbours bacteria which grow and perform biological filtration after a period

of time.

acteria grow on every surface in the aquarium and are fed on the by-products of fish

wastes, excess feeding and fish and plant respiration. iological filtration relies on

colonies of two types of bacteria'Nitrosomonas,which break down toxicammoniaorammonium to nitriteandNitrobacter, which then convert harmful nitrite to a safernitrate

compound (for further information refer to theitrogen !ycle). $t may take six weeks or

more for enough bacteria to develop and accomplish this task successfully and this periodis known as cycling the tank.The filter provides an ideal breeding ground for bacteria as

it is there that a constant supply of food (wastes) and oxygen (conveyed by water flow)

exists. A vast surface area is needed to house the millions of bacteria necessary to

perform efficient nitrification and it follows that the filter media should consist of such amaterial.

iological filtration is not so much a means of cleaning the water, as a processes of

purifying it through the biological conversion of compounds contained within thewater. &imilarly, chemical filtration alters the water properties via a reaction. !hemical

filter media can bond with substances in the water to remove ammonia or ammonium,

change the p" or alter hardness. /ne of the most over looked forms of chemical filtration

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takes place with every water change when most of us add a water conditioner to remove

chlorine or chloramine from tap water. Additionally, the exchange of gases such as the

removal of carbon dioxideand addition of oxygen enhanced by surface agitation from afilter outlet can also be defined under chemical filtration but this is covered in greater

detail in the aerationsection.

y understanding the basic functions of a filter, one is half way to selecting a filtration

system to suit their intended aquarium and its occupants. The next step is to choose afilter of sufficient capacity to cope with the si0e of the aquarium. *ost manufacturers

helpfully specify the si0e of aquarium for which a particular filter is designed but as a

rule of thumb, one should look at the flow rate of the filter. The flow rate refers to theamount of water that can pass through the filter in a given time. $n 1urope this is

specified in litres per hour (lph). This figure should be 2-3 times the volume of the tank

for non or lightly planted aquarium, or 4-5 times the volume for a planted tank. ear inmind that some fish do not relish a current in the water and subsequently filter with a low

rate or one that can accommodate a spray-bar should be chosen. !onversely, for

aquariums which contain large or particularly messy fish a larger capacity filter isrecommended.

!hoosing a filter that is too small for the intended aquarium causes a couple of

undesirable effects. Firstly, the filter may not harbour enough bacteria to process all the

toxic by-products of the itrogen !ycle. Thus the water will become quickly polluted(primarily by with excess ammonia) and poison the fish. &econdly, there is the potential

that a small filter may become rapidly clogged. This not only puts strain on the pump, in

the case of motorised filters, but also restricts the flow of oxygen necessary for the

bacteria to thrive.

As the bacteria dies, it turns anaerobic and inactive in the conversion of toxins. Frequent(but not tougher) cleaning of the filter media can alleviate these problems but why create

work6 !onversely, one can have too much filtration. This is not harmful in itself butunnecessary. $t may also lead the hobbyist into a false sense of security as far as tank

maintenance is concerned. acteria will multiply and grow on as many surfaces as

possible however, there will always be a finite number of bacteria limited by the supply

of food (waste products) and oxygen produced in the aquarium. acteria like to spreadout too. Therefore, if one has a large filter area, the bacteria culture will simply be less

densely populated than in an equivalent set up with a smaller filter area. The secret of

successful filtration is to maximise and maintain the bacteria culture but the very natureof bacteria colonisation means that whenever one cleans any part of the filter media,

regardless of si0e, some of the bacteria population will be reduced. This is why one

should never clean or replace all the filter media at one time. 7oing so will mean thecycling process must be started again.

There are however benefits of having a two or more of the same filter, or combination of

different filters servicing one tank. #hen a number of filters are used, and they are

cleaned in rotation, there less likelihood of depleting the bacteria culture to such extremesas to cause the effect of a newly set up tank. &imilarly if one filter should fail an

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additional filter will act as a reserve. A third advantage is that a number of filters

positioned around the aquarium may service the tank more efficiently in terms of filtering

from a wider area in the tank. Also turbulence can be reduced if a number of smallcapacity filters are used. Finally, a combination of filters can perform different

specialised filtration duties. i.e. one may choose to use a diatom filter, which is purely

mechanical filtration, to polish the water during maintenance, and a very lowmaintenance fluidi0ed bed filter to act as the permanent biological filter.

To assist in choosing a filter, $ have listed below the most popular types of filter. 1ach

link will take you to a description of the selected filter and explain the benefits and

drawbacks of each.

8nderstanding the nitrogen cycle and its role in the aquarium is vital in order to maintain

a healthy environment for your fish.

/nce fish are introduced to a body of water inevitably the initial quality of water will

degrade. Fish excrete, food remains rot and plant leaves decay. The stages by which thesewaste products are broken down by bacteria is termed the nitrogen cycle.

STAGE 1:9otting debris and excreta from the fish produce highly toxic ammonia(or in

acid waters ammonium), which pollutes the water.

STAGE 2:Nitrosomonasbacteria convert the toxic ammonia (or ammonium) into less

toxic, but still dangerously harmful nitrite(/5). itrite is toxic to fish at levels of above4 mg per litre. 1ven in a mature aquarium there will still be a trace (less than 4 mg:ltr or

parts per million) of ammonia (or ammonium) present as the conversion to nitrite is not

an instantaneous one ut as with ammonia, the ideal level of nitrite should be 0ero.

STAGE 3:Nitrobacteract on the nitrite to convert it to less poisonous nitrate(/2).Although nitrate is relatively harmless, long term or sudden exposure to high

concentrations should be avoided. 7epending on the species, fish are susceptible to

nitrate levels above 4;; mg per litre. Above levels of 4

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Nitrobacterbacteria are not immediately present in a new aquarium and must multiply

and develop over time. $t may up to > weeks for enough bacteria to coloni0e before the

nitrogen cycle can be accomplished efficiently. This process is known ascycling thetank.

The water cycle describes the sequence of evaporation and condensation of water in ourenvironment. $t is the sun which provides the energy to initiate the cycle. 1vaporation

from lakes, rivers and the sea carries water vapor into the atmosphere where it cools(condenses) to form clouds of water droplets. /nce the droplets reach the ground as rain,

they percolate through the soil and gather to form rivers, lakes and ultimately oceans

once again.

As water travels around the cycle an number of factors influence its chemical

composition. As rain falls, it can absorb oxygen,carbon dioxideand other gases such as

sulphur dioxide emitted from power stations producing acid rain, and during

thunderstorms rain absorbs nitrogen oxides produced by lightning.

Arguably it is once the rain reaches the ground that the greatest chemical changes occur.

As the water filters through the ground, it becomes charged with carbon dioxide from the

respiration of plant roots and other organisms. $t is usually this that makes water slightly

acidic. 7epending on the ground, other substances will dissolve in the water. #aterdrained from farmland will containnitratesfrom the effects of fertili0ers, filtration

through chalk or limestone causes water to become alkaline and hard and water which

has collected underground is usually high in minerals.

The terrain through which water passes before finally reaching the rivers greatly

influences the water chemistry and thus the conditions in which our fish thrive. $t is

through our water changes and filtrationmethods that the aquarist tries to replicate stagesin the water cycle to create ideal conditions for the fish.

As the biological processes within an aquarium take place, fish and nitrifying bacteria

utilise oxygen and waste gases such as carbon dioxideand nitrogen are produced. $f an

imbalance of harmful gasses occurs, and the oxygen content in the water becomes

deficient, the fish will literally suffocate. An indication of this happening is when fishhang =ust below the water surface and gulp air directly from the atmosphere. (ote? some

fish such as labyrinth fish and Corydoras sp. will occasionally take air from the surface

naturally).

$n an aquarium, the only place that an exchange of gases can occur is where water is indirect contact with the atmosphere. $t is therefore important to maximise the surface area

of the water.

Tall tanks may not have a large enough surface area to provide an adequate oxygeninterchange and in heavily stocked aquariums the demand for oxygen by the fish may

exceed the supply at the surface. $n very warm tanks oxygen diffuses rapidly from the

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water and additional agitation of the water surface will be required to maximise the

surface area available for the exchange of gases.

ormally the return from a filteragitates and circulates the water enough to promotesufficient diffusion of gasses in and out of the water and additional aeration is not

necessary. To address any additional aeration requirements, an airpumpis sometimesemployed. The stream of pretty bubbles which it produces, via an air stone, is often

mistakenly thought to somehow in=ect air into the water but in fact an airpump actuallyprovides aeration by circulating and disturbing the water surface to create a larger area

for oxygen exchange. Too much turbulence at the water surface may drive off carbonic

acid, which is an important plant fertili0er or cause too great a current for the fish. $t is forthese reasons that airpumps and filters are available in a range of si0es or have flow

regulators to tailor their output to the tanks needs.

As the biological processes within an aquarium take place, fish and nitrifying bacteria

utilise oxygen and waste gases such as carbon dioxideand nitrogen are produced. $f an

imbalance of harmful gasses occurs, and the oxygen content in the water becomesdeficient, the fish will literally suffocate. An indication of this happening is when fish

hang =ust below the water surface and gulp air directly from the atmosphere. (ote? somefish such as labyrinth fish and Corydoras sp. will occasionally take air from the surface

naturally).

$n an aquarium, the only place that an exchange of gases can occur is where water is in

direct contact with the atmosphere. $t is therefore important to maximise the surface areaof the water.

Tall tanks may not have a large enough surface area to provide an adequate oxygen

interchange and in heavily stocked aquariums the demand for oxygen by the fish mayexceed the supply at the surface. $n very warm tanks oxygen diffuses rapidly from thewater and additional agitation of the water surface will be required to maximise the

surface area available for the exchange of gases.

ormally the return from a filteragitates and circulates the water enough to promote

sufficient diffusion of gasses in and out of the water and additional aeration is notnecessary. To address any additional aeration requirements, an airpumpis sometimes

employed. The stream of pretty bubbles which it produces, via an air stone, is often

mistakenly thought to somehow in=ect air into the water but in fact an airpump actuallyprovides aeration by circulating and disturbing the water surface to create a larger area

for oxygen exchange. Too much turbulence at the water surface may drive off carbonic

acid, which is an important plant fertili0er or cause too great a current for the fish. $t is forthese reasons that airpumps and filters are available in a range of si0es or have flow

regulators to tailor their output to the tanks needs.

itrosomonas bacteria oxidise ammonia and form the ion nitrite (/5-). #hile not as

toxic as ammonia, nitrite is still undesirable in an aquarium. itrite reacts withhaemoglobin in red blood cells forming methaemoglobin which effectively blocks the

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oxygen uptake to the fish%s bloodstream. The lack of oxygen causes the fish to become

lethargic and stop eating eventually resulting in death. *ethaemoglobin also turns the

blood brown and this is most noticeable in the gills. *any fish keepers may refer to thisas brown blood disease.

@ike ammonia, the ideal level of nitrite in an aquarium is ; mg:l or 0ero parts per million(ppm). &usceptibility depends very much on the fish' some will tolerate levels of up to

4;; mg:l. "owever, levels as low as 5 mg:l will make fish more prone to disease attackand most fish will not survive at this level for more than 3 days.

Arguably (as you will encounter high nitrites during cycling or possibly after filter

maintenance), any reading above 5 mg:l indicates a water quality problem that must becorrected. Although $ mentioned earlier that some fish would tolerate higher nitrite levels,

keeping nitrite to a minimum will ensure that you do not encounter too many problems.

!heck nitrite periodically using an appropriatetest kit.

The final stage of theitrogen !yclein the aquarium (apart from those aquaria with anestablished nitrate removal system) is the breakdown ofnitriteby nitrobacter bacteria to

nitrate.

itrate is certainly less toxic to fish than eitherammoniaor nitrite and until recently

considered to be harmless unless in very high concentrations. &tudies have since shownthat although nitrate may not be lethal unless in exceptionally high quantities it does

make fish more susceptible to disease and reduces the likelihood of breeding success.

The other problem with high nitrates is the possibility of inducing nitrate shock to new

fish when introducing them to a tank containing higher nitrates than they are accustomed

to. As with nitrite, some fish are more sensitive to the effects of nitrate than others.

A level of ; mg:l (0ero parts per million) in a freshwater tank is difficult to achieve

however one must try to maintain as low a nitrate level as possible. !ertainly levels

above 5; mg:l should be avoided for sensitive fish such as 7iscus and levels below 3;mg:l should acceptable for most other fish. !heck nitrates regularly with atest kit.

!ycling the aquarium is the term used to describe the period during which the aquarium

matures and reaches a biological equilibrium from its initial set up. $t is one of the most

critical stages in setting up an aquarium and the most common area of failure forbeginners.

$n order for filters to process the nitrogenous wastes produced in an aquarium (primarily)

by the fish, a colony of bacteria must develop (refer to the section onfiltersand the

itrogen !yclepage for more information). The bacteria%s =ob is to break down highlytoxic ammonia(and ammonium) to less toxic nitriteand then to convert the nitrite into a

safer nitratecompound. $t may take over six weeks to develop enough bacteria to cope

efficiently with the decomposition of wastes formed in the aquarium, to gain the desired

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goal of 0ero ammonia, 0ero nitrite and the presence of only nitrate. #hen this is achieved

the aquarium has reached its biological equilibrium and can be termed mature.

A period of around two weeks should be allowed between filling the tank and introducingthe first fish. 7uring this time, the minimal number of bacteria naturally present in water

must be fed in order to multiply. This can be done in a number of ways. A few handfulsof substrate from a mature disease free aquarium can be added as a bacteria starter

culture, a piece of raw meet can be dropped in and allowed to decay or a small pinch offish food can be added daily. Alternatively, one may use an extremely hardy fish such as

a comet goldfish. our local dealer should be willing to exchange the goldfish once it has

served its purpose. This is also an ideal way to observe the cycling process as one canmonitor the fish%s behaviour.

&pecial additives are also available to speed up the maturation process. +ersonally $

advise against the use of these as the cheaper alternatives outlined above work =ust as

well if not better. The cycling process should not be rushed. Fishkeeping is a hobby

which requires patience, it is not a race - one should master this quality from thebeginning.

7uring the first few weeks, ammonia levels will rise rapidly and then fall to 0ero.

Following the presence of ammonia, nitrite will be produced. itrite will also peak andfall before the final phase of the cycling process occurs, the presence of nitrate. e aware

though, it is not unknown for nitrite to peak more than once before falling to 0ero. $t is

imperative to ensure that the nitrite peak has passed before you add fishes, it may takelonger than two weeks so monitor nitrite every other day during the cycling period. There

is no need to monitor ammonia levels as these will peak before nitrites, which are the

critical factor in establishing whether or not a tank is cycled.

Failure to monitor this process and adding fish too early will result in death of the fish byammonia and nitrite poisoning. This phenomenon is know as ew Tank &yndrome.

The fact that a term exists, indicates that this is an all to common situation.

/nce it is established that nitrite has stabili0ed at 0ero fish can be added. 7o not add too

many fish at once and this applies even after the cycling period. $ never add more thanfive fish in any > week period even in my largest and oldest tanks. The trick to

maintaining a healthy aquarium and avoiding unnecessary deaths is to manage the

bioload placed on the filter. That is to ensure the bacteria colony in the filter can copewill the increase of ammonia and nitrite created by every subsequent fish added to the

aquarium. $t can be a fine balancing act and one which is learned over time but as $

mentioned before patience is the key.

#hen the aquarium is mature and both ammonia and nitrite levels are 0ero, the remainingconcern is the build up of nitrate. Although nitrate is relatively harmless, long term or

sudden exposure to high concentrations should be avoided and this is best controlled via

partial water changes. As a rule of thumb, 5

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should be undertaken to establish when water changes are required to control the rise in

nitrate in your particular set up.

+hotosynthesis is the process by which plants use chlorophyll, with the aid of lightenergy, to synthesis water, carbon compounds, and other nutrients into sugars. These

sugars provide energy for the plant%s growth. $n water, carbon dioxide (!/5) dissolvesquickly and is therefore the simplest carbon compound for plants to utilise. Thus, aquatic

plant life depends on carbon dioxide in the water in order to survive.

As photosynthesis requires light,this process can only occur during the day. $n this

period, plants draw carbon dioxide from the water and release oxygen into the water. At

night however, there is no photosynthesis and the reverse occurs - plants consume oxygenand release carbon dioxide.

$n the average aquarium, light levels are usually low and the amount of !/5produced by

the respiration of the fish during the day is barely sufficient to allow some plants to

photosynthesis and grow. "owever, many plants require more light than is generallyprovided and it is likely that with an increase in light, there will not be enough !/5in the

aquarium to facilitate photosynthesis. +ut simply, the plants can not grow as fast as they

would like to, given the available light energy.

@ight is the first determining factor for photosynthesis and growth. As light increases,plant growth will increase. As plant growth accelerates, !/5will be used faster and

additional quantities may be required. !arbon dioxide without sufficient light will reduce

photosynthesis and cause an excess of !/5to be dissolved into the water.

A balance must be maintained between light levels and the amount of !/5being in=ected

into the aquarium in order for sufficient levels of photosynthesis to take place.

$ncreasing the population of fish (which respire and therefore increase the amount of

!/5) may subsequently provide =ust enough carbon dioxide for the plants in the tank, but

the consequences of overstocking the tank often outweigh any benefits. A largepopulation of fish can produce tremendous amounts of ammoniaand nitrogenous by-

products. oth compounds are usable by plants, but an excess of them can cause

uncontrollable algae blooms and prove to be toxic to the fish. Additionally, as the carbondioxide levels increase, the dissolved oxygen levels decrease (but not to the complete

exclusion of oxygen), making it difficult for the fish to breathe.

A way around the overstocking problem is to in=ect carbon dioxide directly into theaquarium. Anyone who has witnessed the rapid growth of aquarium plants in response tocarbon dioxide (!/5) fertili0ation must be convinced of the usefulness of this system.

&ure, there are thousands of aquarium hobbyists who do not give their plants any sort of

special treatment yet still end up with a fairly nice display. "owever, truly luxuriantgrowth, the sort that you see on the covers of fish keeping aquaria maga0ines can only be

achieved by fertili0ing with !/5.

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$n order to maximise the benefit of in=ecting !/5it is important that you reduce any

surface turbulence, as this will quickly allow dissolved carbon dioxide to escape into the

air. This is one reason why people find it difficult to grow plants if an undergravel filter isemployed - the carbon dioxide is driven from the water.

As a guide, a good level of !/5in an aquarium is around 2< parts per million (ppm). Thiscan be checked with a carbon dioxide test kit or by simply observing the behaviour of any

fish you may have in the tank. $f you see your fish gasping at the surface or breathingrapidly, the level of !/5may be too high (oxygen too low). @evels of 4;ppm !/5

suggested by many, stem from research with coldwater trout and similar fish and have

been carried across to tropical fresh water applications over the mists of time.

!/5concentration must exceed