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You are here: myPetSmart.com > Pet Care Library > Articles > Understanding Aquatic Filtration

Understanding Aquatic Filtration


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Proper filtration is critical to keeping happy, healthy fish. A complete filtration system should neutralize ammonia and nitrites, and remove floating debris and contamination from the water. There are three basic filtration methods; mechanical, biological and chemical. Your filtration system should incorporate all three.

  • Mechanical Filtration
    Mechanical filtration is the means by which large particles of excess food and other debris are removed, screened, or skimmed from the water. This is achieved by flowing water through fiber floss, gravel, foam, or some other screening material.


  • Chemical Filtration
    While mechanical filtration uses filters to remove debris, chemical filtration uses activated carbon and ammonia absorbents, such as zeolite, to remove odor, colors and harmful substances, such as ammonia, from the water.

    With activated carbon, each piece of carbon is like a little sponge that traps odors, medication residue, and dissolved fish waste. Carbon also removes discoloration in cloudy water. Activated carbon will no longer effectively absorb anything and should be replaced after about 3 or 4 weeks. To test the absorption power of carbon, take a glass of aquarium water on white paper. If the paper looks yellow through the glass, it's time to change the carbon.

    Ammonia can be removed by using a liquid ammonia remover that is placed directly in the water, or ammonia chips, which are put into the filter. However, the use of chemicals to remove ammonia should only be required in unusual cases (such as the start up of a new tank or after a large number of new fish have been added). The primary means of removing ammonia and nitrites should always be your biological filter.

    If you need to medicate your aquarium, you'll need to remove the carbon when treating the sick fish. Otherwise, the carbon will absorb the medication.


  • Biological Filtration
    A well-established aquarium is a natural ecosystem in which your fish and the beneficial bacteria that naturally occur in an aquarium depend upon each other to live happily and healthy. The result of this interrelationship is commonly referred to as the "Nitrogen Cycle".

    Fish eat and produce ammonia as a waste product. Excess food and plant materials also decay and produce ammonia. Beneficial bacteria neutralize the ammonia and produce nitrites, which in turn are neutralized by other beneficial bacteria that produce nitrates. Nitrates in normal levels are harmless to freshwater fish. Thus the natural system in your aquarium converts toxic ammonia into harmless nitrates; all without chemicals or your assistance.

    The only thing that you need to do is ensure that you start with a good biological filtration system and that you maintain it. Three conditions are needed in order to establish biological filtration and develop a healthy colony of beneficial bacteria:

    1. There needs to be a place for the bacteria to grow. Bacterial will grow on any porous surface in your tank; on the gravel bed; the replaceable carbon cartridge, wheel, plate or sponge in your power filter; on the bio media in your canister or wet/dry; or on the sand in a fluidized bed filter.
    2. The water must be oxygenated. Bacteria needs oxygen to reproduce and grow. An aquarium with proper aeration of the water and good water flow over the beneficial bacteria, will provide sufficient oxygen to maintain the beneficial bacteria.
    3. There must be a source of food (ammonia) for the bacteria. Any tank with fish or plants will provide sufficient food. The filtration system must circulate the ammonia carrying water over the beneficial bacteria for them to eat.

What choices do I have in filtration systems for my aquarium?
A variety of methods are used to filter an aquarium. All the common methods incorporate all 3 elements of an effective filtration system — mechanical, biological and chemical filtration. The most common methods are:


  • Undergravel filters
    Undergravel filters in which a slotted plate under the gravel bed is used to provide continuous circulation of the aquarium water (either by introducing a stream of air bubbles into the lift tube or using a powerhead to pump the water) down through the gravel and up through a lift tube back into the tank. Biological filtration occurs as beneficial bacteria living in the gravel neutralize ammonia and nitrites as the water passes through the gravel bed.


  • Mechanical filtration
    Mechanical filtration occurs as the floating particles are forced onto the gravel bed and trapped. When using an undergravel filter it is essential that the gravel be vacuumed thoroughly on a regular basis to remove the trapped particles of food and other waste. Also trapped waste that accumulates under the filter plate needs to be cleaned periodically. Failure to vacuum the gravel or clean under the filter plate can result in changes to your water chemistry that could be harmful to your fish.

    A typical undergravel filter provides no chemical filtration. However, a carbon or zeolite (to remove ammonia) cartridge can be added to the lift tube to provide the needed chemical filtration. These cartridges contain small amounts of carbon or zealot so they need to be changed frequently to be effective.


  • Power filters
    Power filters have become the most commonly used filtration system in tanks up to 55 gallons. Many hobbyists use a power filter along with an undergravel filter to increase the biological filtration (and thus the number of fish they can keep) in smaller tanks. And many use power filters on even larger tanks in conjunction with canister and other filters.


  • External power filter
    An External power filter is the best choice for combining chemical, mechanical and biological filtration with ease of use.

    An external power filter hangs on the back of your aquarium and is basically an electric pump that draws water from your aquarium and pumps it through a replaceable filter cartridge that is typically filled with activated carbon. The "carbon cartridge" provides the chemical and mechanical filtration. Biological filtration is accomplished by passing the water over a wheel, sponge, or porous plastic plate that houses the beneficial bacteria.

    In some cases the beneficial bacteria live on the replaceable cartridge. You should only use a filter designed in this way with an undergravel filter as other biological filters since replacement of these cartridges removes the beneficial bacteria from your system.


  • Canister filters
    Canister filters are a very effective means of providing a total filtration system. Canister filtration is most typically used on 55-gallon aquariums and larger. While some canister filters are designed to hang on the back of your aquarium, most are designed to be put under the tank and hidden in the aquarium stand. Many believe that an advantage of the canister filter is that it is hidden from sight and thus is more aesthetically pleasing. A major advantage of using a canister filter is the flexibility it gives you in adapting the filter to your other mechanical, biological and chemical filtration needs. These filters are designed to let you determine how much (and what type) of filtration media you want to use.

    The only disadvantages are that it requires more hoses and connections than an external power filter and thus is more complicated to set up. Also this type of filter must be disassembled to change media and thus is somewhat more difficult to maintain. Recent designs from the major manufacturers have significantly improved the ease of set up and maintenance of these types of filters.

    Other filtration methods have been developed over the past few years. Wet/dry filtration and fluidized bed filters are two of the more popular methods. Both methods can be very effective biological filters. However, they must be used as part of an overall filtration system. Remember, when picking the filtration for your aquarium, you need 3 types of filtration - mechanical, biological and chemical, all working together to have the happiest and healthiest fish.

How Do I Introduce Beneficial Bacteria into My Aquarium?
Beneficial bacterial will develop in your aquarium as soon as you introduce a food source (ammonia from fish waste or decaying excess food as plant material). You can speed up the process by putting a few cups of gravel from a healthy, established aquarium into your new aquarium. Commercial bacteria starters are also available that can help speed up the development of the beneficial bacteria.

We recommend that you gradually add fish to any new aquarium. Give the bacteria a chance to keep up with the fish. It may take at least a month for the bacteria to develop sufficiently to control the ammonia and nitrites in a fully stocked aquarium.

During the period that your beneficial bacteria are developing, we suggest that you test the water frequently to insure that the ammonia and nitrites levels are not too high. High levels of these chemical can have a serious effect on your fish health. A freshwater test kit will prove to be an important tool in helping you be successful with your new aquarium.

Why Should I Do Regular Water Changes?
Partial water changes are an essential part of maintaining good water chemistry. Nitrates are the natural result of the "Nitrogen Cycle" which occurs in your aquarium. While nitrates are normally harmless to freshwater fish, they can be detrimental if allowed to build up. Partial water changes are the best way to ensure you don't have a build up of nitrates. We recommend that you change 20% to 25% of your water every 2 weeks.

An efficient way of maintaining your aquarium is to vacuum the gravel while doing a partial water change. You can eliminate any decaying food or other debris that builds up in the gravel while assuring that you control the nitrite levels in your aquarium. There are several types of gravel vacuums that make this process fast and easy.

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