The living machine
A filter is doing three different jobs at once, and only one of them is the one most people picture. Understand the three, and you will know which parts you may clean freely, which you must treat as living, and why "a clean filter" can be a contradiction in terms.
Reading time ~10 minutes · Pairs with Water Chemistry
Three jobs, not one
Filtration in an aquarium is conventionally divided into three functions, usually arranged in sequence so the water passes through each in turn.[8] They are not interchangeable, and a filter that does only one of them is only doing a third of the work.
1 · Mechanical filtration — removing particles
This is the job most people imagine: physically straining solids — uneaten food, fish waste, plant debris — out of the water so it runs clear. Sponges, foam pads and filter floss do this. Mechanical media is the part of the filter you can clean often, and should: trapped detritus that is left to break down simply adds to the ammonia load.[8] But clean it in removed tank water, not under the tap, because mechanical media also hosts bacteria.
2 · Biological filtration — the part that actually keeps fish alive
This is the heart of the filter and the reason the whole device exists. Across the surfaces of porous media — ceramic rings, sintered glass, mature sponge — live the nitrifying bacteria that run the nitrogen cycle: ammonia-oxidisers converting ammonia to nitrite, and Nitrospira-like bacteria converting nitrite to nitrate.[2][1] They form a biofilm and need oxygen and a steady supply of their food (ammonia and nitrite), which is why flow must be maintained and the colony never starved or sterilised.
Biological media is engineered for enormous internal surface area, because the size of the bacterial colony — and therefore the tank's capacity to detoxify waste — is limited by the area available for it to grow on.[8] "More biomedia" really does mean "more processing capacity," up to the limit of what your flow and waste load support.
Replacing all the biological media at once, or rinsing it in chlorinated tap water, removes the colony and can crash an established tank back into a toxic, "uncycled" state.[2] If you must replace biomedia, do it a portion at a time over several weeks so the colony can re-grow on the new surface before the old is gone.
3 · Chemical filtration — optional, and worth understanding
Chemical filtration removes dissolved substances by adsorption — most commonly with activated carbon, whose vast internal pore structure binds organic molecules, some medications, and the compounds that cause discolouration and odour.[9] It is genuinely useful for polishing water or stripping out a medication after treatment, but it is not essential to a healthy tank, and it has limits: carbon saturates and stops working, so it must be replaced periodically or it becomes merely an extra piece of mechanical media. Other specialist chemical media exist for particular jobs (for example, resins that bind phosphate or ammonia), each targeting a specific problem.
| Stage | Typical media | Servicing |
|---|---|---|
| Mechanical | Sponge, foam, filter floss | Rinse often in tank water; replace floss when clogged. |
| Biological | Ceramic rings, sintered glass, mature sponge | Disturb as little as possible; never sterilise; replace only in stages. |
| Chemical | Activated carbon; specialist resins | Replace on saturation; optional in a healthy tank. |
Sizing, flow, and the reserve sponge
A filter must move enough water to keep the whole volume turning over and to supply the biological colony with oxygen and waste to process; an undersized filter leaves dead spots and an overloaded colony.[8] One quietly excellent habit, borrowed straight from the Founding Researcher's day job, is to run a second, spare sponge somewhere in the system. It matures into a fully colonised biological filter you keep in reserve — an instant, seeded backup for a new tank, a hospital tank, or the day your main filter fails. It is resilience built before it is needed.
Read next
- Water Chemistry — what the biological stage is actually doing, in detail.
- The Complete Method — how to service a filter during routine maintenance.
- Algae Management — when "polishing" the water with chemical media helps, and when it hides the real problem.