Inline Heater Guide
How external inline heaters warm water in a canister filter return line, keeping hardware out of the display, and how to install and run them safely.
What it is
An inline heater is an external heater installed in the tubing that carries water back from a canister filter or sump to the aquarium. The heater body sits outside the tank, so no heating hardware is visible inside the display. As water flows through on its way back to the tank, the inline heater raises its temperature.
How it works
Water leaving the canister filter or sump passes through the inline heater before returning to the aquarium, where it is heated to the set temperature; once the target is reached the element switches off. Because the warmed water is then dispersed through the tank by the return outlet and any pumps, heat spreads evenly. For larger aquariums this even distribution helps avoid cold or hot pockets caused by uneven flow.
Pros and cons
- Pro: no heater hardware inside the display tank, giving a clean look.
- Pro: even heat distribution through the return flow, useful for larger tanks.
- Con: requires a canister filter or sump with external plumbing.
- Con: more complex to install than a submersible heater.
- Con: depends on continuous flow; loss of flow risks overheating without proper protection.
Choosing and sizing
Inline heaters are aimed at medium to large aquariums that already use a canister filter, broadly from roughly 100 litres upward. The heater's rated wattage should match the tank volume and the required temperature rise, following the same volume-based sizing used for submersible heaters.
Installation
The heater is installed on the output (return) line of the canister filter or sump, not on the intake, because post-filter water is far less likely to carry debris that could clog the unit. It is generally mounted vertically to avoid trapped air pockets and dry spots. Connections must be tight to prevent leaks, and the heater should include overheat protection or automatic shut-off in case flow stops.
Maintenance
Because the heater is plumbed into the filter loop, it is typically serviced alongside the canister filter. Connections are checked for leaks and the unit cleaned during filter maintenance. A typical service interval for this configuration is roughly every six months.