Hang-on-Back Filter Guide
How hang-on-back (HOB) power filters work, their strengths and limits, and how to size, install, and maintain them for small to medium aquariums.
What it is
A hang-on-back (HOB) filter, also called a power filter, is an impeller-driven unit whose body hangs on the outside of the aquarium rim while an intake tube reaches into the tank. Water is drawn up the intake by the motor, passed through a stack of filter media inside the box, and returned to the tank, typically as a small waterfall over a spillway. It is one of the most common filter types for small and medium aquariums.
How it works
The three modes of aquarium filtration are mechanical, chemical, and biological. Mechanical filtration removes particulate material such as uneaten food, feces, and debris by passing water through mesh-like media. Chemical filtration removes dissolved wastes, commonly using activated carbon. Biological filtration relies on nitrifying bacteria that drive the nitrogen cycle: ammonia from waste is converted to nitrite by Nitrosomonas, then to less toxic nitrate by Nitrospira and Nitrobacter. An HOB filter passes water through media chosen to deliver all three modes before returning it to the tank.
Pros and cons
- Pro: media selection is flexible, so mechanical, chemical, and biological stages can be combined.
- Pro: the box hangs outside the tank, so it can be opened and cleaned without disturbing the inhabitants.
- Con: media capacity is smaller than a canister filter.
- Con: it can create excessive flow in very small tanks.
- Con: vibrations and the waterfall return can be noisy, and the motor can burn out if it runs dry.
Choosing and sizing
Hang-on-back filters suit small to medium aquariums, broadly in the range of roughly 20 to 200 litres. Because they generate a steady current, they may produce too much flow for the smallest tanks unless the output is baffled. The media basket capacity should be matched to the bioload, since smaller boxes hold less biological media than a canister.
Installation
The body hooks over the rear rim with the intake tube lowered into the water. The unit must be filled or primed so the impeller does not run dry, as a dry motor can burn out. The waterfall return aerates the surface; lowering the water level slightly increases splash and surface agitation.
Maintenance
An external box makes routine service straightforward. Mechanical media and sponges should be rinsed periodically, and chemical media such as carbon replaced on a schedule. To preserve the bacterial colony, biological media should be rinsed gently rather than replaced wholesale, since the bacteria perform the nitrogen cycle. A common service interval for this filter type is about monthly.