Filtration
Hang-on-back filter
The practical middle option. Servicing takes seconds, the return creates good surface agitation, and media is easy to customise. The weaknesses are noise as the water level drops, a narrow media volume, and manufacturers' cartridge systems that encourage throwing away the biological filter every month.
Short answer
A pump lifts water into a box hanging on the tank rim, through media, and returns it over a weir. Convenient, easy to service, and the most common filter on small and medium tanks.
What it does
- Mechanical, biological and chemical filtration in one unit
- Strong surface agitation from the return flow
Strengths and where it fits
Strengths
- Easy access for maintenance without disturbing the tank
- Good surface agitation and gas exchange
- Inexpensive and widely available
- Media can be replaced with sponge and ceramic instead of disposable cartridges
Good fit for
- Community tanks up to about 150 L
- Keepers who want easy servicing
- Tanks needing good surface movement
Limitations and where it is the wrong choice
Limitations
- Limited media volume compared with a canister
- Noisy as evaporation lowers the water level below the return
- Unprotected intake can trap fry, shrimp and long fins — fit a pre-filter sponge
- Disposable cartridge systems discard biological capacity if followed as instructed
Poor fit for
- Very large tanks
- Rimless tanks where the unit is visually intrusive
- Shrimp tanks without an intake guard
Sizing
- Rated flow is measured with no media and no head. Choose a unit rated for more than the tank volume — commonly one and a half to two times the nominal requirement.
- Ignore the instruction to replace cartridges monthly. Rinse sponge in tank water and replace only when it falls apart.