The purpose of filtration is to maintain water quality and sustain the life of the inhabitants. The process of filtration is divided into three types – mechanical, chemical and biological. All three types of filtration must be present and working properly to maintain the water quality necessary to support life in a freshwater or marine aquatic environment. Therefore, routine maintenance is necessary to keep these filter systems working optimally.
Mechanical filtration involves the removal of particulate matter suspended in the aquarium water. If these suspended particles are not removed, the water will become cloudy in appearance. Mechanical filtration is accomplished in the filter bed using foam inserts, filter floss, or other particulate trapping material.
Excrement from vertebrates and invertebrates, aerobic respiration byproducts from vertebrates, the natural decay of plant material and the decay of uneaten food particles all create waste products, namely nitrogen based compounds, which accumulate in the water. Natural biological filtration is the process by which specific types of bacteria detoxify, or break down, various nitrogen based compounds in the water into less harmful compounds.
This breakdown of nitrogen compounds is known as the nitrogen cycle and it is believed that two major types of beneficial bacteria, Nitrosomonas and Nitrobacter are the primary bacteria involved. The majority of these beneficial bacteria colonies are found clinging to bio-balls in the bio-filter, the gravel bed substrate and various aquascape substrates such as rock. These beneficial bacteria are not found in high quantities in the water column.
Chemical filtration is the process by which dissolved organic compounds are removed from the water. Although organics are the primary materials removed by chemical filtration, other substances such as some nitrogen compounds can also be removed this way.
Chemical filtration may utilize specialized filter media and/or specialized equipment. Examples of specialized filter materials include activated charcoal and ion exchanging resins, whereas Protein Skimmers*, Ozone Generators, and UV Sterilizers are all examples of specialized equipment that remove specific organic compounds. *It should be noted that Protein Skimmers are very unique devices in that they achieve both mechanical and chemical filtration.
Protein Skimmers, or foam fractionators, are primarily used in marine systems and not freshwater systems because the specific gravity, or density, of freshwater, is not great enough for protein skimmers to work properly. There are a four design types: Venturi, co-current (unidirectional flow of air and water), counter-current (opposing flow) and Down Draft. One of the most popular Protein Skimmers recommended for marine aquariums is the Venturi driven Protein Skimmer, because it is an efficient means for removing dissolved and small particulate organic compounds. For marine systems over 300 gallons, a Down Draft Protein Skimmer is considered best suited because greater volumes of water can be exposed to filtration. One downside with Down Draft Protein Skimmer is their size. They may be too large for some residential or commercial aquariums.
One downside with all Protein Skimmers is that they remove trace elements, but these trace elements can easily be replaced with water additives.
Ozone Generators create very small quantities of ozone gas, O3, a poisonous gas. Ozone oxidizes, or breaks down, various unwanted chemical compounds and destroys disease-causing microorganisms floating in the water column.
Ozone generators are most useful in aquarium systems that house large numbers of fish and where there is a need to control the amount of rapidly accumulating dissolved organics and microorganisms. Because ozone can be toxic, even in small quantities, these systems should be used with extreme caution are by experienced aquarists.
As water passes through an Ultraviolet Sterilizers, the water is irradiated with high-frequency UV light, invisible light in the blue part of the spectrum. UV Sterilizers are effective in destroying both good and bad free floating bacteria, and disease causing viruses, fungi, microorganisms and parasites. UV Sterilization is fairly common today in aquariums and is one of the most effective methods for control of disease causing organism and parasites.
Major Types of Filter Equipment
There are five primary different types of filter systems used by aquarists: Undergravel Filters, Wet/Dry Filters, Hang-On Power Filters, Canister Filters, and Foam Filters. Each filter system has its pros and cons and one is usually more appropriate depending on the type and size of aquarium system.
These filters fit underneath the gravel bed and are constructed of thin, raised perforated plates. Hundreds of tiny slits in the perforated plates allow water to enter, but block larger materials and particles such as crushed coral or gravel from entering. Pumps force water to circulate from the tank into the undergravel filter, which acts as both a mechanical and biological filter. One benefit of undergravel filters is that they prevent anaerobic bacteria from developing in the gravel bed. But on the other hand, undergravel filters have a tendency to become clogged and occasionally must be removed for cleaning which is no easy task. One major downside with cleaning undergravel filters it that it disrupts the biological aspect of these filters.
The most popular filters used in marine aquatic systems are Wet/Dry filters. They are called Wet/Dry filters because unlike other filter systems, the filter media in Wet/Dry filters is not fully submerged in water, rather, it is kept moist by continuous water spray or drip. Basically, water in the aquarium display drains over a spillway tube or wall inside the aquarium and exits through holes drilled in the aquarium bottom. As the water exits the aquarium, it is directed by plumbing into the Wet/Dry Filter. There the water trickles over mechanical filtration material and then through a bio-filter chamber filled with bio-balls. The water then is usually channeled through one or more different chemical filtration devices before it is pumped back into the main body of the aquarium. One advantage to Wet/Dry filter is that as the water trickles through different parts of the filter, significant amounts of dissolved oxygen is introduced into the water. In fact this is one of the best means for saturating water with dissolved oxygen. Wet/Dry filters have tremendous surface areas ideal for harboring large colonies of the beneficial bacteria that help keep an aquarium in balance. One disadvantage is that the trickling water in Wet/Dry filters can be considered noisy by some. Overall, many consider Wet/Dry filtration as the best filter technology available today.
Hang-On Power Filters
Hang-On Power Filters are designed to hang on the back side of the aquarium. They are powered with a centrifugal pump, therefore an air pump is not required to oxygenate the water. Water drawn into the filter is passed through a series of compartments that house the mechanical, chemical and bio-chemical filters. These systems are usually very simple and are well suited for small freshwater aquarium systems.
Canister Filters are very similar in principle to Hang-On Power Filters, but are freestanding and can be placed inside an aquarium cabinet out of site. Water from inside the aquarium is forced into the bottom of the canister under pressure where it passes through the various filter layers and exists out the top of the canister. The water is then returned to the aquarium. These units are compact, very quiet and can be stored out of sight. They are suitable for both freshwater and marine systems. These filtration systems have had a reputation for being expensive since they were primarily made overseas, but today, more domestic units being manufactured, thus lowering the retail cost of these units. Canister filters do have disadvantages. First, these filters dissolve less oxygen compared to other filters. Secondly, maintenance on these filters disrupts the biological aspect of the filter.
Foam or Sponge Filters
Foam or Sponge Filters are constructed of porous materials that perform biological and mechanical filtration. This type of filtration systems uses synthetic materials with large surface areas that allow the colonization of large numbers of nitrifying or beneficial bacteria. Forced water is filtered as it flows through the porous filter material. Foam or Sponge Filters are primarily suited for small aquariums.
Filter materials are the various materials found in mechanical, chemical and biological filter compartments. Some materials are spent after use, others can be used continuosly.
Bio-balls are openly designed plastic balls designed to maximize surface area for allowing beneficial bacteria to grow. Bio-balls are found in the biological filter compartment. Once in place, bio-balls should be left undisturbed and should always be wet. Cleaning bio-balls is never recommended because this will kill beneficial bacteria colonies.
Activated Carbon, Ion-Exchange Resins
The products are part of the chemical filtration system and remove certain organic compounds from the water. These products work best if the water is forced through these filtering materials. Typically these products are spent after use and must be replaced approximately every 30 days.
Filter Floss is one of the most commonly used materials in mechanical filtration. The fibrous wool-like material is excellent at trapping small un-dissolved free floating particles. Filter Floss should be replaced as needed.