Aquarium filters keep your tank clean and healthy. Choosing an aquarium filter that is right for your situation is important. The type and size of your filter system will vary, depending on factors such as the type of aquarium (freshwater or saltwater), the size of the tank, and the type of inhabitants. The three basic types of filter systems are biological, mechanical, or chemical. Your filtration system may use a single type of system, but most likely will employ a combination of these types.
When choosing
an aquarium filter, consider a biological filter. Biological filters work to break down dangerous and undesirable compounds like ammonia, nitrite, and nitrate by introducing other biological elements or processes. They keep your water stable and free of toxins. Common types of biological filters include under gravel filters, sponge filters, canister filters, trickle filters, and power filters. The most common biological filtration system for aquariums is an under gravel biological filter. Water is pulled either down through the gravel or (with a reverse flow filter) down from the top of the tank and then up through the gravel.
Sponge filters are usually powered by air pumps or small water pumps and pass water over porous sponges. These are a good option for tanks with very small fish that may be in danger of being sucked into larger filters. Canister filters hold various filter media inside. The actual type of media used will depend on the specifics of your tank. Trickle filters hold biological media outside the water and allow the tank water to trickle over the media and back into the aquarium. Power filters, like canister filters, are primarily mechanical in nature but also include a biological component.
Mechanical filters use a fine-gauge cartridge or other medium through which water is forced. This filter cartridge itself traps impurities. Mechanical filters keep the water looking clean by removing debris. Common types of mechanical filters are cartridge, power, 
and under gravel filters. These are most beneficial when used in conjunction with biological filters. Using a mechanical filter alone will keep your aquarium water clean, but will also remove some of the elements you need, such as healthy algae, to keep your water safe.
Another option to consider when choosing an aquarium filter is chemical filters. Chemical filters work to remove elements that cannot be removed by mechanical filters because the elements have dissolved into the water. Activated carbon is the most common type of chemical filtration, normally used in conjunction with a power or canister filter. Activated carbon will remove many potentially harmful substances and help keep your tank water looking clear. Other common chemical filtration agents specifically remove or reduce the levels of nitrates, phosphates and ammonia.
When choosing an aquarium filter, be sure to carefully evaluate the needs of your tank system. Having an aquarium filter that is not sufficient for the size and population of your tank can result in harmful byproducts being left in the water. These can sicken or even kill you fish. At the same time, choosing an aquarium filter that is significantly more powerful than your system demands can mean healthy bacteria are being stripped from your water.
