To clean a hang-on-back filter, unplug it, remove the media basket or cartridges, rinse reusable sponges and biological media in old aquarium or dechlorinated water, and remove sludge from the filter box, intake tube and impeller. Replace only media that is exhausted or physically worn out. Reassemble the filter, refill the box with aquarium water if needed and confirm normal flow after restart.
Hang-on-back filters often come with cartridge replacement schedules that encourage owners to throw away the most biologically mature part of the filter. In many setups, preserving reusable media is safer and cheaper.
Turn the filter off before opening it
Unplug the unit and remove it or open the lid according to the design. Never put fingers near an impeller while the filter is powered.
Rinse reusable media instead of sterilising it
Squeeze sponges and gently swish biological media in old aquarium water or dechlorinated water. Brown staining is normal and does not mean the media must be replaced.
Clean the intake and impeller
Plant fragments, snails and biofilm can restrict the intake. Pull the impeller and clean the chamber if flow has fallen or the motor is noisy.
A small brush is useful for intake tubes and corners.
Be careful with disposable cartridges
If the cartridge is mainly mechanical and still intact, it may be rinsed and reused. If it also contains carbon that you specifically need, replace that chemical component when exhausted rather than automatically discarding every mature surface.
Restart and confirm siphon or prime
Some HOB filters need the box filled with water before restart. Once running, check that the waterfall flow is normal and the motor is not rattling or running dry.
Frequently asked questions
How often should I clean a HOB filter?
When flow decreases or debris accumulates. Many need monthly to every few months depending on stocking and media size.
Should I replace the cartridge every month?
Not automatically. Reusable media can often be rinsed and kept; follow the actual purpose and condition of the media.
Why is my HOB filter noisy after cleaning?
Check that the impeller is seated correctly, the filter is fully primed and no air is trapped.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.