Filtration

    There are three basic classifications of filtration; biological, chemical and mechanical. However, the lines separating each type can get a bit fuzzy. Bacteria, a major source of biological filtration, often live in and on mechanical filters. Many filtration units (such as canister filters) combine mechanical and chemical filtration. Protein skimmers could be considered mechanical or possibly chemical filters depending on how you look at them. In any case, here is a basic over view of these three different kinds of filtration:

Biological Filtration

    Biological filtration involves bacteria, algae and other live organisms involved in nutrient cycling and fixation. Bacteria and algae are everywhere in a reef tank (even where you can't see them). They're in the water, on the glass, rocks and even in and on components of other filters.

Good News: It's very easy to get more biological filtration. Generally, anything with surface area through which water can flow will add biological filtration to your tank. Over time, filter floss, sponges, and other materials used for mechanical filtration, often become sites for biological filtration as bacteria populate the surfaces of these materials. Additionally, sand beds can potentially add incredible amounts of surface area for biological filtration. However, to make sure that water gets through your sand bed, you'll need to make sure your sand bed is set up and maintained properly (see Sand Bed Basics). Your biological filter(s) will also respond to increasing and decreasing food availability. The more you feed your tank, the more biological filter you will "grow." This is essentially what's happening during algal blooms. However, keep in mind that your biological filter will not always be able to keep up with sudden increases in feeding levels or livestock loads. Increases in feeding and livestock additions should always be slow and gradual.

Bad News: As mentioned, your biological filtration can take the form of algal blooms that you may not appreciate. Also, since it can't be "seen" functioning or failing to function, it can be difficult to immediately ascertain the health or capacity of your biological filter.

Chemical Filtration

    Probably the most common and wildly used type of chemical filtration is carbon. Carbon is a porous matrix of microscopic "holes" that trap (filter out) a variety of different chemicals, including many substances toxic to reef invertebrates (even many of those produced by corals). Other types of chemical filtration include phosphate "sponges" and other media designed to filter out specific chemicals and toxins.

Good News:Carbon is a great filter medium so long as you get the right kind of carbon (coal carbon, preferably cracked, virgin coal carbon). It can help spare your tank from some of the toxins produced by leather corals, dying xenia, and other coral-produced poisoned. It clarifies the water and can remove overdosed aquarium medications. Some kinds of phosphate sponges and other chemical filters can be useful as well. However, they will not suffice as your only form of filtration or only method for phosphate/nutrient removal.

Bad News: Any given amount of carbon can only filter out so much. If not changed regularly, carbon (and/or any other media which acts as a chemical "sponge") can get saturated and then even start to leak compounds back into the water. The same goes for almost all chemical filter mediums.

Mechanical Filtration

    Mechanical filtration is the physical removal of particulate matter from the water. There are a variety of different kinds of mechanical filters. Sponges and pressure filters are both common types of mechanical filtration. Diatom filters could also be considered mechanical filters. Diatom filters can filter out particles even too small to be seen with the naked eye, including some life stages of the Ich parasite.

Good News: Mechanical filtration is a fast, easy, and indiscriminate way to get "stuff" out of the water. It will take out excess food and other floating particles. Diatom filters can help clean up in-water-column algae blooms and even the swimming stages of the marine Ich parasite.

Bad News: Once food, waste, and other dead, degrading organic matter decomposes into water soluble compounds, mechanical filtration will not be able to remove them. Thus, intermediates of the nitrogen cycle, dissolved phosphates, and other dissolved organics are not removed by mechanical filtration alone. This is why biological and chemical filtration methods are also needed.

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Other Forms of Filtration and Combination Filters:

Protein Skimmers

    Protein skimmers form lots of bubbles out of the hydrophobic compounds in the aquarium water. These bubbles form a foam and are collected as "skimmate." The foam is pushed up the column of the skimmer and trapped in the "skimmer cup." The skimmate that collects in the skimmer cup contains a myriad of organic compounds and chemicals, most of which are partially dissolved organic proteins.

Good News: Protein skimmers can be very efficient at removing the semi-dissolved organic matter that chemical and mechanical filtration fail to remove. They can also help remove some forms of phosphates and heavy metals. Additionally, protein skimming can help oxygenate aquarium water.

Bad News: Protein skimming is a fairly indiscriminate form of filtration. While it does remove a lot of undesirable compounds and substances, if may also remove food sources for some corals and filter feeders. Protein skimmers should probably be turned off during and for several hours following feeding. Also, while protein skimmers can help oxygenate water, by this same mechanism they can also introduce indoor air pollutants. Therefore, it's wise not to smoke or spray chemicals in the room with your aquarium.

Note: For more on protein skimming, please see Protein Skimmer Q&A.

Canister Filters

    You can sometimes spot a convert from freshwater aquarium keeping by their unwillingness to let go of their beloved canister filters. These filtration units are a tried and true classic for all types of aquarium keeping. When filled with activated carbon, they combine mechanical, biological and chemical filtration all in one unit.

Good News: Canister filters are quite versatile. You can fill them with just about any filtration media. You can even use diatom powder with most of them (provided the right membrane), turning them into make-shift diatom filters.

Bad News: For reef aquariums, canister filters are great to have on hand, but are not sufficient by themselves. They can also be quite a hassle operate and keep clean.

Wet/Dry Filters

    Wet/Dry filters are designed to be primarily biological filters (most also incorporate a mechanical filter as a "pre-filter"). Basically, these filters work by flowing water over filter balls (or other shape/type of specifically designed filter pieces) that are only partially submerged in water. The idea is that this "wet/dry" environment provides an ideal environment for a set of beneficial bacteria for biological filtration. This may or may not be true. Certainly, bacteria do grow on the filter balls (as bacteria will grow anywhere). However, if this environment is ideal for precisely the kinds of bacteria that make for the best biological filter is still largely unknown.

Good News: For whatever reason, these filters seem to work well for fish-only tanks.

Bad News: It appears that, over time, wet/dry filters fail to keep nitrate levels down low enough to maintain a healthy reef aquarium. Though some reef aquarists do still report success with them, recent trends have steered reef keepers away from these filters.

Zeolite Filtration

    The idea to use zeolite as an aquarium filtration media initially came from its use in municipal water and sewage treatment. By definition, zeolite is "any of a large group of minerals consisting of hydrated aluminosilicates of sodium, potassium, calcium, and barium. They can be readily dehydrated and rehydrated, and are used as cation exchangers and molecular sieves." In more ordinary language, zeolite is a type of porous rock that can be thought of as a lattice or matrix of minerals that may or may not be chemically charged. In water, they can sometimes be used to selectively filter out specific chemicals like ammonia. However, there are (at least) many hundreds of different kinds of zeolite rocks. Each one will behave differently in water and pull out different chemicals to different degrees. There's also the theory (though it's never been proven) that these porous rocks can selectively grow the types of bacteria most beneficial to aquarium biological filtration. In larger industries, some zeolites are man-made and designed to specifically filter out certain chemicals. In the aquarium industry, the type and nature of the zeolite rocks sold as filter medium are largely unknown.

Good News: Some aquarist swear by zeolithic filters, claiming that filtration with a zeolite product "saved" or otherwise greatly improved the health and life of their aquariums.

Bad News: Virtually nothing is known about the zeolites sold for reef aquarium filtration. There's no real proof that this type of filtration works for reef aquariums. In other words, use at your own risk.

UV Filtration

    UV filters use UV radiation to denature organic compounds (including DNA) in the water. They also kill many smaller organisms and larvae.

Ozone

    As the name implies, ozonaters inject ozone (O3) into the aquarium water. Ozone oxidizes ammonia and nitrites into less toxic compounds.