How to clean an aquarium filter

Your filter does two jobs. It traps dirt, and it's home to the good bacteria that keep ammonia at zero. Clean it the wrong way and you can throw those bacteria away. Done right, it takes ten minutes.

How often?

Clean the filter when the flow slows down, not on a strict schedule. For most hang-on-back and internal filters that's every 2–4 weeks. Canister filters can usually go 1–3 months.

The three golden rules

• Rinse media in old tank water, never under the tap. Chlorine in tap water kills the good bacteria. Clean the filter on water change day, so you have a bucket of old tank water ready.
• Don't replace all the media at once. If something needs replacing, change one part at a time, a few weeks apart.
• It doesn't need to look new. A sponge that's a bit brown is full of the bacteria you want. You're only clearing the clogs.

Cleaning a hang-on-back filter, step by step

1. Unplug the filter.
2. Fill a clean bucket with water from the tank. Use a bucket that has never had soap in it.
3. Take out the media and put it straight into the bucket.
4. Squeeze the sponge in the bucket water a few times until water runs through it easily.
5. Swish the ceramic rings or bio media gently. Only rinse them if they're clogged with gunk.
6. Clean the impeller. Pull out the intake tube and the small spinning magnet underneath. Wipe off any slime with a soft brush or cloth. A slow or noisy filter is often just a dirty impeller.
7. Put everything back in the same order.
8. Fill the filter box with tank water before you plug it in. Many hang-on-back filters won't start pulling water if they start dry.
9. Plug it in and check that the water is flowing well.

Sponge and canister filters

Sponge filters: lift the sponge out, squeeze it a few times in a bucket of tank water, and put it back. That's it.

Canister filters: switch off and close the valves, then open the canister over a bucket or sink. Rinse each tray of media in tank water, check that the rubber seal is clean and seated, put it back together, and prime it the way your manual describes.

What about filter cartridges?

Many filter boxes say to replace the cartridge every month. That throws away most of your good bacteria each time, which can cause an ammonia spike. Instead, rinse the cartridge in tank water and keep using it until it starts falling apart. When you do replace it, leave the rest of the filter alone that week. If your filter has room, adding a piece of sponge or some bio media gives the bacteria a permanent home.

Carbon

Carbon removes smells, tints and medicines from the water. It's optional in most tanks. If you use it, replace it about once a month, and always take it out while you're treating fish with medicine.

Mistakes to avoid

• Rinsing media under the tap.
• Cleaning the filter and deep-cleaning the gravel on the same day. Spread big cleans out so you don't remove too many bacteria at once.
• Restarting a filter that has been off for hours without rinsing it. The water inside goes stale. Rinse the media in tank water first.
• Forgetting to test afterwards. Check ammonia a day or two after a big clean.

Related: How to do a water change · Ammonia in a fish tank · Cloudy aquarium water