Blog
Filter Media Explained: Mechanical, Chemical and Biological
Open any filter and you will find some combination of sponge, floss, carbon and ceramic. They look like variations on a theme. They are not — they do three separate jobs, they wear out at three different rates, and treating them the same is the single most common reason a cycled tank crashes.
Mechanical media: catching the solids
Mechanical media is a physical sieve. Coarse sponge catches the large debris, fine floss polishes out the particles that make water look hazy. Neither removes anything dissolved; they simply hold solids until you rinse them out.
This layer always goes first in the flow path. Put it anywhere else and the finer media downstream clogs with detritus that should have been caught at the door. Coarse before fine, always — that ordering doubles the interval between cleanings.
How often to clean it
Rinse mechanical media whenever flow noticeably drops, typically every two to four weeks. Rinse it in a bucket of tank water you have just siphoned out, never under the tap: chlorinated tap water kills the bacterial colony living on the sponge. Fine floss is the one item here that is genuinely disposable — once it stays grey after rinsing, replace it.
Chemical media: pulling things out of solution
Activated carbon is the common one. It adsorbs dissolved organics, removes discolouration and odour, and strips out medications after a treatment course. It works quickly and then it stops working — the surface saturates, usually inside three to four weeks.
Two things worth knowing. First, exhausted carbon does not release its load back into the water in any meaningful way, but it does become just another lump of mechanical media, so leaving it in indefinitely gains you nothing. Second, carbon will strip liquid medication and some plant fertilisers out of the water, so pull it during a treatment course and put it back afterwards.
Other chemical media exist for specific problems: resins for phosphate, ammonia-binding zeolite for emergencies, peat for softening. All of them are targeted tools. If you do not have the problem, you do not need the media.
Biological media: the part that must not be replaced
Ceramic rings, sintered glass and porous blocks exist to give nitrifying bacteria somewhere to live. Those bacteria convert ammonia to nitrite and nitrite to nitrate; they are the entire reason an established tank is safe for fish and a brand-new one is not.
This is the layer people destroy by accident. Throwing away the biomedia along with the dirty sponge, or rinsing it under the tap, removes the colony and the tank re-cycles from scratch — ammonia spikes, fish stress, and a fortnight of daily water changes. Biological media should be rinsed gently in old tank water only when it is visibly clogged, and otherwise left alone for years.
If you must replace it
Replace at most a third at a time and leave the rest in place for at least a month so the new material can be colonised from the old. The same rule applies when moving to a bigger filter: run the old media inside the new unit for a few weeks before retiring it.
The cartridge question
Many hang-on-back filters ship with a single all-in-one cartridge: floss on the outside, carbon inside. It is convenient and it works, with one catch — the instructions usually say to replace the whole cartridge monthly, and the cartridge is also carrying part of your bacterial colony.
The practical compromise is to keep a small permanent sponge or a handful of ceramic media in the same chamber, so that when the cartridge goes the colony does not go with it. Most units have room for it. Our filter media and cartridges listings state which units each refill fits, and the hang-on-back filters we stock are chosen partly on whether refills are easy to get.
A maintenance rhythm that works
- Every 2–4 weeks: rinse the coarse sponge in old tank water; replace fine floss if it stays grey.
- Every 3–4 weeks: renew carbon if you are running it.
- Every 6–12 months: inspect the impeller and shaft, clean the housing, check the O-ring on a canister.
- Almost never: touch the biological media.
Stagger the jobs. Rinsing the mechanical media and renewing the carbon in the same session is fine; doing that and disturbing the biomedia in the same week is asking for a spike.
Reading the water
Cloudy water shortly after a media change usually means a bacterial bloom, and it clears on its own. Cloudy water with a rising ammonia reading means the colony took a hit. Persistent haze with normal parameters is usually a mechanical problem — fine particles passing straight through — and is solved with a floss pad rather than a chemical.
If you are not sure which of those you are looking at, describe the symptom and your last few maintenance jobs and we will help you narrow it down. You can also check the FAQ for the questions that come up most, or browse the filtration range.
From our shelves
Products in this store that relate to what is covered above:
Prices are in US dollars. Delivery timings are on our shipping policy page and the return and refund policy sets out the window if something is not right.
