Inline Heaters Explained: External Temperature Control Without Clutter

Most hobbyists start with a glass heater sitting inside the tank, held by a suction cup, doing its best not to get knocked around by fish or aquascape rearranging. Inline heaters take a different approach entirely: they sit outside the display, plumbed directly into the water return line from a sump or canister filter, heating water as it passes through rather than sitting in the tank itself.

How an inline heater actually works

An inline heater is a sealed chamber, usually PVC or a similar rigid plastic, with a heating element inside and barbed or threaded fittings on either end. Water flows through the chamber on its way back to the display, picking up heat as it passes the element. There is no glass to crack against a rock, no cord trailing through the display, and no unit for territorial fish to knock loose. The Hydor 200 Watt Inline Heater is a good example of this format, built to be plumbed inline on the return side of a sump-fed system rather than dropped into the tank.

Because the heater sits in the sump area or under-cabinet plumbing rather than in the display, it is effectively invisible once installed. For anyone running a planted tank, reef, or biotope where clean sightlines matter, that alone is a strong argument for going inline.

Sizing wattage to volume

Wattage needs to match total system volume, not just the display. If you have a 300 litre display with a 100 litre sump, you are heating 400 litres, not 300. As a rough starting point for Brisbane conditions, where ambient room temperature rarely drops as hard as it does further south, allow around 1 watt per litre for reef and tropical freshwater setups running standard tropical temperatures. Colder rooms, glass ambient rather than insulated cabinets, or larger day-night swings might need a bit more.

• Small systems (under 150L total volume): 100 to 150 watt units are usually enough.
• Mid-size systems (150 to 400L): 200 watt inline heaters, like the Hydor unit, sit comfortably here.
• Larger systems (400L plus): consider two heaters split across the return plumbing or sump, rather than one oversized unit, for redundancy and more even heat distribution.

Running two smaller heaters instead of one large one also means that if one fails, the tank does not swing hard in temperature before you notice. That redundancy matters more in reef systems where a couple of degrees of drift can stress corals.

Where to plumb it for even heating

Inline heaters should sit after the pump, on the pressurised side of the plumbing, so water is actively pushed through the chamber rather than relying on gravity or siphon flow. Placing it close to the return pump outlet, before any spray bars or manifolds split the flow, gives the most even heat distribution across the whole tank rather than one loop running warmer than another.

Orientation matters too. Most inline heaters are designed to run horizontally or at a slight upward angle so the chamber stays fully flooded and air cannot pocket around the element. Check the instructions for your specific unit and mount it in a spot that is easy to isolate with unions or ball valves either side, because at some point you will want to service or replace it without draining the sump.

Pairing with a thermometer and controller

An inline heater still needs monitoring like any heater. A simple glass or digital probe from our thermometer range gives you a manual check, while a dedicated controller lets you set tighter tolerances and get an alert if temperature drifts outside the range you expect. Either way, do not assume