Hydor ETH 200 Inline Review
An external heater that clips into your canister filter's return hose, warming water as it flows back — so nothing hangs inside the tank at all.
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👍 Pros
- Sits outside the tank on the filter's return line — zero clutter inside the glass
- Self-limiting PTC element cannot exceed its rated surface temperature — inherently safe
- Warms water evenly as it passes through, avoiding a single hot spot
- Adjustable temperature dial on the external body, easy to reach in the cabinet
👎 Cons
- Only works with a canister filter (or a pump) pushing water through it
- Pricier than an in-tank heater of the same wattage
- Must never run without flow — always wire it so it can't heat a stopped filter
Heating without the tube
The Hydor ETH solves a problem every aquascaper feels: the ugly heater tube stuck to the inside glass. Instead of sitting in the tank, it splices into your canister filter’s return hose and warms the water as it flows back, so the heater lives in the cabinet and the display stays clean. It was the first external aquarium heater of its kind, and its self-limiting PTC element is the clever part — it physically cannot climb past its rated surface temperature, which makes a thermostat runaway essentially impossible.
Fit, flow and sizing
Because it works on flow, the ETH only suits tanks running a canister filter (or a dedicated pump). Match the model to your hose diameter — the common 200 W unit fits 12 mm / half-inch tubing — and to your volume: at the usual 1 watt per litre guide, 200 W comfortably covers a 150–200 L tank. The one rule you must respect is flow: never let it heat a stopped filter, so wire it to the same power as the pump. It only ever heats moving water.
Where it fits with our other heaters
An inline heater is a lifestyle choice — you accept a higher price and a canister-only requirement in exchange for a totally equipment-free tank. If you would rather keep things simple in-tank, the Eheim Jager 150W and shatter-resistant Aqueon Pro 150W are cheaper and self-contained. On a tank over 200 L, add a second heater for redundancy as covered on the aquarium heaters hub. It pairs naturally with a good canister on our filters hub — match both to your tank on the aquariums page.
The aquascaper's heater. If you run a canister filter and want a completely equipment-free tank, the ETH hides the heater in the hose and its self-limiting element makes it one of the safest ways to warm the water.
Hydor ETH 200 Inline — frequently asked questions
How does an inline heater work?
It splices into the flexible hose that carries water from your canister filter back to the tank. As the filter pushes water through the ETH's chamber it is warmed on the way in, so the heater lives in the cabinet and nothing hangs inside the display. It only heats when water is flowing, which is why it must be plumbed on the return line of a running filter.
What size tank and hose does the ETH 200 fit?
The 200 W version matches the common 12 mm / half-inch canister hose and, at the honest 1 watt per litre rule, comfortably heats a tank around 150–200 L. Check your filter's hose diameter before buying — Hydor makes the ETH in different bore sizes, and it must match your tubing to seal properly.
Is it safe if the filter stops?
The ETH uses a self-limiting PTC element that physically cannot exceed its rated surface temperature, so it will not run away like a stuck thermostat. Even so, best practice is to power it from the same socket as the filter (or via the filter's timer) so it can never sit heating a chamber of still water. On big tanks, a second heater still adds useful redundancy.
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