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Aquarium Manual
Aquarium Filtration



Filtering Aquarium Water - Good Water is Synonymous with Good Health

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For aquatic animals, good water quality is as important as good air quality is for human beings. Aquarium water quality is dependent on efficient filtration of the water in your tank. So, understanding filtration is one of the most important tasks facing the new aquarist.

Even the novice aquarist is aware that high levels of ammonia will quickly spell the end to her fish. Unfortunately, many water quality problems are more subtle. There is a domino effect set in motion with deteriorating water quality, ending in the death of a tank full of pets.

As water quality slowly worsens, the fish suffer from physical stress. Stress, over time, will cause the health of the fish to decline. Chronic stress makes fish vulnerable to bacterial and fungal infections and parasitic infestations. These infections and infestations will be but the conclusion to the phenomenon that began with poor water quality caused by inadequate filtration.

An appropriate filtration system and good aquarium management can minimize, if not completely eliminate, chronic stress. To better illustrate the correlation between fish health and filtration, a discussion of the three basic types of pollutants that accumulate in aquarium water and the corresponding filter options for each kind is appropriate.

Particulate Pollutants: Filter Out Debris and Particles Mechanically

There are a number of sources for the particulate matter that accumulates in aquariums: uneaten fish food, digested waste products, and cell material from fish and plants. Some of this material remains in suspension in the water rather than sinking to the bottom of the tank, making the water "turbid" - cloudy and dirty.

Larger particles sink to the bottom, only to rise again and cloud the water when disturbed by fish or aquarium maintenance activities.

Besides being aesthetically unappealing, fish and plants are adversely affected buy turbid water. Turbidity-induced stress weakens the ability of fish to resist the disease organisms that are a normal part of the aquatic environment. Gill filaments may become irritated and begin to swell interfering with respiration, which heightens stress, and eventually causes gill membranes to become susceptible to bacterial and parasitic invasion. Deposits of particulates on plant leaves reduce light adsorption and gas exchange, and many plants will die as a result. Finally, suspended particulates may also support blooms of disease-causing bacteria.

Daily water changes of about one-fourth of the tanks water and and in-tank gravel cleaning, would reduce particulate pollution. Who has time for that? (Not to mention, too much new water can be quite stressful to some fish.)

The best way to deal with suspended material in the water is with mechanical filtration. This works by collecting particles from the water as it continuously circulates through a mechanical screening medium such as nylon or polyester floss, foam sponges, fiber pads, aquarium sand and diatomaceous earth.

These media differ primarily in terms of the size particles they can effectively capture, their cost, service life, and re-usability. At one extreme is floss, which tends to trap only the largest particles and cannot be reused, but is inexpensive. At the other extreme is diatomaceous earth, which can trap extremely fine particles (including bacterias, parasites and algae spores)

The size of the spaces - or pores - in the media, the shape of the spaces, and the roughness of the material determine the particulate-trapping ability of mechanical filtering media.

Filtering medium that traps very fine particles will quickly clog and requires frequent replacement. Otherwise, the water flow will be restricted to the point where turbidity in the tank actually increases! A better method is to use a coarser filter material that traps larger particles first , such as coarse fiber pads. Then a finer filter pad for finer particles. If only one pad is used, the accumulation of larger particles fills in the spaces, and smaller particles are then trapped. But you run the risk of smaller particles just by-passing the clogged media pad.

An important factor in choosing the mechanical filter is the rate at which the water will flow through the material. An outside-the-tank power filter should circulate (in gallons per hour) four to six times the volume of the aquarium in gallons. Thus, for a 50-gallon tank, the filter should process at least 200 gallons per hour.

When choosing from among several filters of the same flow rate, note which design offers the largest filtering-medium surface area.

Organic Pollutants: Proteins and Other Organic Wastes

A variety of substances such as proteins, amino acids, and other metabolic by-products are continuously being discarded into the aquarium water by the fish. These organic substances are dissolved into the water - thus the name, dissolved organic carbon (DOCs).

If these DOCs are allowed to become concentrated in the water, the health of the fish will suffer.

The practical way to deal with DOCs is by chemical filtration techniques that are reliable and economical. The most common and reliable are carbon filtration and protein skimmers (foam fractionation).

Carbon filtering removes DOCs via ADsorption. As the aquarium water flows through the carbon medium, the DOCs come into contact with the surfaces of the carbon granules and become attached. Some DOC compounds are adsorbed directly, whereas others combine chemically with already-captured substances.

NOTE: When the surfaces of the carbon become saturated, it must be discarded and replaced with new carbon or it can start to throw its trapped compounds back into the water.

Many types of carbon are sold for aquarium use, but only a few are actually capable of adsorption in water. The material of choice is high quality granular activated carbon (GAC). This is carbon that has been de-gassed in an oxygen oven at temperatures close to 2,000 degrees Fahrenheit. The de-gassing enlarges the apparent surface area of each carbon granule, greatly increasing the adsorptive capacity.

In a filter, the GAC (carbon) should always be placed after the mechanical filtering medium. Otherwise, the particulates in the water will quickly coat the surfaces of the GAC, rendering it useless. Deep beds of GAC are always more effective than shallow ones of equal area. Using the smallest granules available maximizes adsorption.

There should be 4 to 10 grams of GAC for every gallon of water, and it should be replaced monthly. Choose the best quality GAC you can find. Any chamber that houses less that 4 grams per gallon of water is useless and will only work to restrict water flow.

Note: Do not use powdered activated carbon. It compacts too easily and seriously restricts water flow except in specially designed systems.

Protein Skimmers bubble water up carrying proteins off the surface of the aquarium and trapping the protein foam in a little container before they get a chance to decay and create ammonia. The most common use of a protein skimmer is in saltwater aquariums, but they work just as well in freshwater tanks. The only difference is that the freshwater aquarium must be set up and running for a while before a protein skimmer works as designed.

GAC beds and protein skimmers remove such substances as hydrogen sulfide, and other toxic chemicals that may find their way into the aquarium. Unfortunately these devices are incapable of telling the bad guys from the good guys and will also remove beneficial chemicals, such as many medications.

Therefore, it is imperative to remove all GAC from the filter and shut off a protein skimmer before medicating fish.

Biological Pollutants: Ammonia, Nitrite, Nitrate

Biological filtration works to eliminate the presence of ammonia and nitrite in the aquarium water. Ammonia and nitrite probably account for more health problems and deaths with aquarium fish than any other cause. The fish excretes (directly through the gills) approximately 60 percent of the ammonia in an aquarium. The remainder is produced by bacterial breakdown of fish feces and urine, uneaten food, dead animals and plant by-products. Nitrite is a waste product of the bacteria that use the ammonia for food.

The accepted theory is that ammonia in aquarium water hinders oxygen uptake in the fish's blood. Lethal concentrations cause chronic stress. Gill filaments become irritated and begin to swell. This cuts off the oxygen supply to the membrane cells, when they become infected with bacteria. Ammonia-related stress also lowers a fish's resistance to parasite and bacteria attacks, reduces food intake, and can result in death. An ammonia test kit measures total ammonia, which in a well-managed aquarium should be immeasurable. All aquarists should have an ammonia test kit and use it on a regular basis.

Likewise, Nitrite toxicity involves blocking oxygen uptake in the blood. As nitrite levels increase, the concentration of methemoglobin (as a percentage of total hemoglobin) goes up, reducing the capacity of the blood to carry oxygen. Sub-lethal concentrations produce chronic stress similar to ammonia. Here again, regular use of a nitrite test kit is an absolute requirement for a fish keeper.

These are all big biological terms to say that ammonia and nitrite kill fish by whatever mechanism. And both are produced by the regular cycle of fish living, breathing, excreting and bacteria eating waste and producing nitrite.

There are two basic solutions to this problem: biological filtration and ion exchange filtration.

Biological filtration is a method that uses naturally-occurring nitrifying bacteria to detoxify nitrogenous wastes. One type of bacteria converts ammonia to nitrite. Another converts the nitrite into a far less harmful form of nitrogen waste: nitrate. Laboratory studies have shown that nitrates are far less harmful to fish than once believed. Concentrations in excess of 400 mg/L appear safe for fish. (Nitrates are more toxic to marine invertebrates.)

These bacteria are usually well established in aquarium gravel, on tank walls and on plant surfaces within two weeks of adding fish to a new set-up. However, although all aquariums have some amount of natural biological filtration occurring on the surface of the gravel bed, it is usually far too little to handle the fish load. A dedicated biological filter is significantly more effective. It will give the bacteria the optimal surfaces to live and multiply.

There are also bacteria enhancers that you can purchase to pour into the tank and seed the bacteria bed quickly and easily - establishing a healthier tank in a smaller amount of time.

Note: Take care not to sterilize or "over-clean" your biological filter bed. While mechanical filters can take a good amount of cleaning and sterilizing, the bacteria are delicate and will die if you use harsh chemicals or chlorinated water to flush or clean them.

Ion Exchange is a chemical filtration process that removes ionized ammonia (NH3+) from the water by swapping it for an ion in the exchange medium. Both natural zeolites (which look like little chips of cement) and synthetic resins (which look like little plastic beads or chips) are used to remove ammonia from aquarium water. Ion-exchange resins (but not zeolites) also remove nitrites and nitrates.

The exchange medium is placed in a compartment of an outside filter (after the compartment with GAC so that the water flows through the GAC first) and aquarium water is forced through it. Ion-exchange media can only be used in freshwater tanks. This also precludes the use of salt in freshwater tanks for medicinal purposes when using ion exchange. Ammonia ion exchange is also severely inhibited in hard water. Even moderate levels of DOC in the water significantly reduce ammonia ion exchange. Thus, the use of zeolites or ion exchange resins requires continuous filtration of particulates and DOCs.

Note: When ion-exchange material becomes saturated with ammonia, it must be recharged or replaced. Otherwise, the ammonia concentration will quickly build up to dangerous levels (known as ammonia break-through). Therefore, the use of ion-exchange media requires regular and frequent ammonia testing.

Strive to use a filter that has all three types of filtration in one unit.

See information about how to choose the right type of filter for your tank.