In short
- Never replace all filter media at once.
- Replace one third of the filter media every 60 days.
- After 180 days, all media have been replaced.
- Replace filter floss as needed, at the latest every 28 days.
- Never wash filter media under tap water.
What a filter really does
An aquarium filter has two jobs. Mechanical filtration traps visible dirt. Biological filtration is done by bacteria living on the filter media, which convert toxic ammonia (NH3) into nitrite (NO2) and then into much less harmful nitrate (NO3).
The second job is the one that keeps animals alive, and it depends entirely on the bacteria that have settled on the media over weeks and months.
What happens when you replace everything at once
New filter media are clean – and empty. If you replace all media in one go, most of the bacteria leave the aquarium with the old media. The waste keeps coming, but nothing processes it. Ammonia and nitrite rise, and the tank behaves like a new, uncycled aquarium with animals already in it.
The rule of thirds: one third every 60 days
Tank Manager plans filter media in thirds. Every 60 days you replace one third of the media with new media and leave the other two thirds in the filter. The next third follows 60 days later, so after 180 days all media have been replaced – but never all at once.
- Two thirds of the bacteria always stay in the filter.
- The new third is recolonised from the media around it before the next third is due.
- The filter never loses its biological capacity all at once.
Filter floss: replace as needed, at the latest every 28 days
Filter floss (fine filter wool) is the exception. Its job is mechanical: it traps fine particles and clogs quickly. Clogged floss slows the flow through the whole filter, so it is replaced as needed – as soon as it is visibly clogged, and at the latest every 28 days.
How to service a filter safely
- Take some water from the aquarium into a bucket.
- Rinse the media you are keeping gently in that aquarium water – never under tap water, which can harm the bacteria.
- Replace only the third of the media that is due with new media. Never replace all media at once.
- Replace the filter floss if it is due or clogged.
- Reassemble the filter and restart it promptly. The bacteria need a constant flow of water.
- Measure ammonia (NH3) and nitrite (NO2) over the following days to confirm the filter is coping.
Signs the filter needs attention sooner
- The flow from the filter outlet is clearly weaker than usual.
- The floss is visibly clogged.
- Ammonia or nitrite appear in your measurements.
Frequently asked questions
Why should I never replace all filter media at once?
Because the media carry most of the bacteria that convert toxic ammonia and nitrite. Replacing all of them at once removes the biological filter and can send the aquarium back to the start of cycling.
How often should filter media be replaced?
One third every 60 days, so all media are replaced over 180 days. The other two thirds always stay in the filter so the bacteria can recolonise the new part. Never replace all media at once.
How often should I replace filter floss?
As needed – as soon as it is visibly clogged – and at the latest every 28 days. Floss is a mechanical medium and clogs quickly.
Can I wash filter media under tap water?
No. Rinse the media gently in water taken from the aquarium. Tap water can harm the bacteria living on the media.
What should I do after servicing the filter?
Restart the filter promptly and measure ammonia (NH3) and nitrite (NO2) over the following days. Both should stay at zero.
This guide is general information for aquarium keepers. It does not replace advice from a veterinarian or other qualified specialist for a specific problem. Last updated: 2026-11-14.