Warning: You still need to do partial water changes, even if you have a filter. Do not add salt to your tank if you are using zeolite filter media. Use a sponge (made for aquarium filters) over the filter intake to prevent curious fish from getting stuck to it. Follow the instructions for your filter on setup and maintenance.
Mechanical Filter
Most people think of an aquarium filter as a pool filter, that basically sucks up and filters out pieces of debris from the water. This is called a mechanical filter and the only function that most basic aquarium filters perform. You will need to occasionally replace or rinse the debris out of whatever media is in the filter. Don’t rinse it with chlorinated water if you’re trying to establish a nitrogen cycle.
Chemical Filter
A chemical filter uses media like activated carbon (charcoal) to filter toxins out of the water. This mostly removes heavy metals, some fish medicine, and tannins from driftwood. This may also contain zeolite which binds ammonia in the water (until exposed to salt). This is frequently the second stage of a two-stage filter.
Biological Filter
A biological filter simply runs water over microbes, usually nitrifying bacteria. This section of the filter will contain porous material that the bacteria can grow on. You generally don’t want to rinse this media, but if you do, be gentle and use aquarium water. This is frequently the third stage of a three-stage filter.
Recommendations
There are filters that hang on the back (HOB) inside the tank, and canister filters, where just the hose goes into the tank. You will probably want an external canister-type filter if you have a tank over 10 gallons. Additionally, if you want a bare bottom tank (without any gravel for bacteria to grow on) you can upsize to a four-drawer filter and have twice the bio media in the filter. Optionally, you can switch out the chemical or mechanical drawers for bio media ones.
Do I need a filter for my aquarium?
No. Aquarium filters are an advanced feature and not a basic necessity. You still need to do partial water changes even if you have a filter. You only need a filter, specifically a biological one, if you want to establish a nitrogen cycle and are using a bare bottom aquarium (an aquarium without gravel or substrate). A filter can improve the appearance of water in between partial water changes. Unfortunately, a common mistake is people believing that the filter prevents them from needing to do partial water changes to maintain water quality that is favorable to fish.