- Mechanical filtration. This takes out the chunks of fish poo and uneaten food before they can settle into the gravel to rot.
- Chemical filtration. This keeps the water from becoming foul smelling and discolored.
- Biological filtration. This is super important because it keeps the ammonia and nitrite levels from becoming toxic - your biological filter is made up of the beneficial bacteria in your fish tank (the bacteria you're trying to establish so that your tank is considered cycled).
The best way to keep your fish healthy and alive for their full lifespan is to provide them with a source of clean, filtered water - poor water quality is the underlying culprit behind a lot of fish problems that new and experienced aquarists are faced with. I personally use AquaClear and Marineland filters on almost all of my fish tanks, with the exception of the larger ones that require canister filters.