Fish release waste into the same environment they eat, breathe and live in, making an efficient filtration system critical to their long-term health and well-being. Choosing the best filter for your aquarium will depend on aquarium size, the types of fish you keep, your feeding habits, maintenance practices and to some extent your personal preferences.
Most filters on the market are rated for specific aquarium sizes, however, the bio-load in your aquarium is just as important if not more so. Simply stated, this refers to the number and size of fish and the amount of food being fed each day. For example, a 55 gallon aquarium with one or two large predatory fish may require a larger filter than the same sized aquarium with dozens of small schooling fish because predatory fish produce larger amounts of waste. Fish that are fed three times a day create more waste – or a higher bio-load – than fish that are fed once a day. For best performance, always choose a filter rated at least one size larger than your aquarium. For aquariums 100 gallons or larger, multiple filters may be required.
Stages of Filtration
There are three stages of filtration: mechanical, chemical and biological. Most aquarium filters perform all three but are sometimes better at one or two at the expense of the others.
Mechanical: The removal of solid waste, organic debris and other particulate matter by trapping it on fibrous or sponge material and then rinsing or replacing the media. This is typically the first stage of filtration, although ultra-fine media for "water polishing" is often placed at or near the end of the flow path in canister filters. The density of the material will determine what size particles are filtered out and the resulting water clarity. Finer media provides clearer water, but usually needs to be cleaned or replaced more often.
Chemical: The adsorption of dissolved pollutants using granular materials such as carbon, ion exchange resins, zeolite and other media. Carbon also removes the yellow or greenish tint common in mature aquariums. Specially treated pads that can be cut to size are also available and provide both mechanical and chemical filtration. Chemical filtration is typically the second or third stage in the filtration process but can vary depending on personal preferences and philosophies. This media must be replaced when exhausted or saturated.
Biological: The conversion of toxic ammonia to nitrite, and then nitrite to nitrate through oxidation by nitrifying bacteria, often known as the Nitrogen Cycle or Biological Filtration. These bacteria grow on permanent media which are usually not changed or replaced. Biological media can be composed of ceramic, sintered glass, plastics or even sponge. The best bio-media have very high surface area for maximum bacterial growth. Biological Filter media is generally the last stage of filtration allowing the beneficial bacteria to have the cleanest water possible.
External Power Filters
Also known as hang-on-back (HOB) or simply hang-on filters, these are the most popular filters for small to mid-size freshwater aquariums because of their reliable performance and convenient maintenance. As their name suggests, they hang on the back of the aquarium and water is drawn or pushed into the filter chamber where it passes first through a replaceable carbon filled fiber cartridge and then some type of permanent biological media before returning to the aquarium. The discharged water agitates the surface, thus oxygenating the water and provides circulation within the aquarium. Some HOB filters can be manually loaded with individual media for more specialized use. Cartridges should be rinsed as needed and changed monthly. Most hang-on filters must be primed by filling them with water before plugging them in, however, Aqueon® QuietFlow LED PRO and the Coralife® Marine Filter with Protein Skimmer are all self-priming. QuietFlow LED PRO filters also feature an LED filter cartridge change indicator, which turns on when water flow indicates the cartridge needs replacing or rinsing, and Specialty Filter Pads for additional chemical filtration.
Compared to HOB's, canister filters hold more media and offer the aquarist unlimited flexibility in terms of media options. They are typically used on mid-size to larger aquariums in both freshwater and saltwater applications. Canister filters are positioned below the aquarium, usually inside the cabinet stand. Water is drawn into the filter through an intake tube, passes through the media and is then pumped back to the aquarium via a return tube. A spray bar or directional jet is used to agitate the surface and provide current in the aquarium. Canister filters function best as mechanical filters and are ideal for large aquariums, those with fish that create a lot of waste or fish that like strong current. Aqueon® QuietFlow® Canister Filters come in 3 sizes and feature a unique water return option, the Water Polishing Unit which serves as the final stage of filtration.
Internal Power Filters
These filters are placed inside the aquarium and are driven by an integrated pump. Open style filters attach to the aquarium glass by suction cups or hang on the rim using clips. Water is filtered through a standard HOB filter cartridge. Closed style internal filters pass water through mechanical, chemical and biological media chambers before directing it back into the aquarium through a controllable directional nozzle. Aqueon QuietFlow E and QuietFlow Internal Power Filters are available in multiple sizes. QuietFlow Internal Power Filters can be installed vertically or horizontally and have adjustable flow rates and directional control, making them ideal for paludarium or riparium use.
Sump or Wet/Dry Filters
These filters were initially developed for the marine hobby, but can be used in freshwater systems, as well. Placed below the aquarium, water is gravity fed from the aquarium down to mechanical filtration medias, such as a fiber sock or other filter pad material and into the first chamber of the sump. From there, water is distributed over biological media where carbon dioxide is released, and the water is oxygenated. Additional chambers can be used for heaters, chemical filtration media, protein skimmers (used in marine aquariums only) and/or other purposes, depending on the needs of the aquarium. After flowing through all the chambers, water is pumped back to the aquarium. There are many different variations of sump filtration systems, but the basic function is the same no matter the design.
Sponge filters are placed on the bottom of the aquarium. Using an air pump, water is drawn into the sponge and ejected through an exhaust tube. Some mechanical filtration occurs, however, their main function is as a biological filter. Due to the high surface area in the sponge, they filter fairly large volumes of water relative to their physical size. While not often used in display aquariums, many breeders use them in nursery tanks because they will not trap baby fish and they support important micro-organisms that serve as first food for newly hatched fry