Canister Filter Guide
How external canister filters work, their large media capacity, and how to choose, plumb, and service them for medium to large aquariums.
What it is
A canister filter is an external filter, usually a sealed cylinder or box that sits below the aquarium and is connected by intake and return hoses. Water is drawn into the canister, travels through several trays of filter media, and is returned to the tank. The aquarium, intake pipe, filter interior, and return pipe form a continuous body of water, so the intake and return siphons counterbalance and reduce strain on the pump regardless of height.
How it works
Water enters through the bottom intake, passes through the chosen media stack, and returns via the top return pipe. The stacked trays let a single unit carry mechanical media to trap debris, chemical media such as activated carbon to remove dissolved waste, and a large volume of biological media to host the nitrifying bacteria that convert ammonia to nitrite and then to nitrate. Because the body holds a high volume of media without occupying space inside the tank, it is well suited to larger and heavily stocked aquariums.
Pros and cons
- Pro: large media capacity for all three filtration modes, producing very clean water quietly.
- Pro: external plumbing allows in-line installation of accessories such as heaters and CO2 diffusers.
- Con: higher cost and greater complexity than other filter types.
- Con: maintenance requires near-complete disassembly, and hoses can be hard to clean.
- Con: sealed media outside the tank can suffer bacterial die-off during prolonged power outages, and there is a small leak risk.
Choosing and sizing
Canister filters are aimed at medium to large aquariums, broadly from roughly 100 litres upward and serving very large tanks. Their large media volume makes them appropriate for heavily stocked or messy fish. Matching the unit's rated capacity to the tank volume and bioload provides headroom for biological filtration.
Installation
The canister sits in the cabinet below the tank, with the intake and return pipes secured to the rim. Connections must be sealed to avoid leaks, and the system is primed before the pump runs. Because the plumbing is external, a heater or CO2 diffuser can be added in line on the return so no extra hardware sits inside the display.
Maintenance
Service is less frequent but more involved than for a hang-on-back filter, since the canister must be disconnected and opened. Mechanical media is rinsed and chemical media replaced, while biological media is rinsed gently to preserve the bacterial colony. A common service interval for this filter type is roughly every two months.