How to clean an aquarium filter
AquaLog guide · Published
In short: Clean the filter when its flow slows, typically about once a month. Rinse sponges and media gently in a bucket of old tank water, never under the tap, and never replace all of the media at once. Every few months, clean the impeller and tubing, then restart the filter straight away.
Why does it matter how you clean it?
Your filter does two jobs. It traps dirt, and it houses many of the bacteria that turn ammonia into nitrate. Clean it too hard and you throw the bacteria away with the dirt, and ammonia can come back. The aim is to restore the flow, not to make the media look new. See how to cycle an aquarium for why those bacteria matter.
Two things harm them: chlorine or chloramine from tap water, and a lack of oxygen while the filter sits switched off. Everything below avoids both.
How often should you clean it?
Go by flow, not the calendar. When the outflow weakens noticeably, the media is clogging. The maintenance checklist suggests a rinse about once a month, but a lightly stocked tank may go longer, and a busy one, especially with messy fish such as fancy goldfish, may need it sooner. Don't leave it too long: water channels past clogged media, leaving airless pockets where the bacteria can't work.
Avoid cleaning the filter on the same day as a big water change or a deep gravel vacuum. Spread the jobs out so the bacteria aren't disturbed all at once.
What does each type of filter media do?
| Media | What it does | How to clean it | When to replace it |
|---|---|---|---|
| Sponge or foam | Traps dirt and holds lots of bacteria | Squeeze in old tank water until it runs mostly clear | Only when it falls apart or won't spring back; one piece at a time |
| Filter floss or wool | Catches fine particles | Too fine to rinse well; lift out the dirtiest layer | When clogged, a little at a time |
| Ceramic rings, sintered glass, bio-balls | Gives bacteria a large surface to live on | Swish gently in tank water, only if coated in sludge | Rarely; only if crumbling |
| Activated carbon | Removes toxins and colour from the water | Can't be rinsed back to life | As the product label says, or leave it out |
| Cartridges (floss plus carbon) | Mechanical and chemical filtering in one | Rinse in tank water and reuse | When it falls apart; if the filter holds two, never change both together |
Carbon also takes medicines out of the water. If you need to treat your fish, check the medicine label on whether to remove carbon first.
How do you clean a filter, step by step?
- Siphon a bucket of water from the tank, using a bucket kept only for the aquarium. Top the tank back up with dechlorinated water.
- Switch off and unplug the filter before you open it.
- Take out the media and put it straight into the bucket so it stays wet.
- Squeeze sponges a few times and swish other media until the worst of the sludge is gone. No soap, no hot water and no tap water.
- Replace any floss or carbon that is due, but never more than part of the media at once.
- Wipe out the housing, and clear the intake and any pre-filter sponge.
- Reassemble, prime it if it is a canister or hang-on-back filter, and switch it on straight away. Check the flow and look for leaks.
- Test ammonia and nitrite over the next few days. Any reading above zero means the bacteria took a knock: feed lightly and change some water. The water change calculator shows how much.
How do you clean the impeller?
The impeller is the small spinning magnet that drives the water. Hair, snail shells and slime build up around it and slow the flow even when the media is clean. Clean it every few months, or sooner if the filter starts to rattle or grind.
- Unplug the filter and remove the impeller cover. Your filter's manual shows how.
- Ease the impeller out. Some sit on a thin ceramic shaft that snaps easily, so handle it gently.
- Rinse it in tank water and clean the well it sits in with a small brush.
- Check the blades and shaft. Chipped blades, a worn shaft or constant rattling mean it's time for a replacement part.
- Refit it, making sure the shaft sits properly in its rubber mounts, then restart the filter.
While the filter is apart, run a long flexible brush through canister hoses and intake tubes. Gunk builds up inside them and cuts the flow too.
What mistakes crash the cycle?
- Rinsing media under the tap. Chlorine and chloramine kill the bacteria.
- Replacing all the media at once. You throw away most of your bacteria in one go.
- Leaving the filter off for hours. Bacteria in still water run out of oxygen. Restart it as soon as it's back together.
- Doing everything on one day. A filter clean, a big water change and a gravel vacuum together disturb too much. See how often to change aquarium water.
- Medicines. Some, especially antibiotics, harm filter bacteria. Test more often after treating.
In a shrimp tank, fit a sponge over the filter intake so baby cherry shrimp aren't pulled inside.
AquaLog logs each filter clean in a couple of taps and reminds you when the next one is due, so you can see your real cleaning interval next to your water changes. Start your log.