Hydor Theo 200W Review
A no-nonsense Italian-made submersible heater with a PTC ceramic core and a shatter-resistant body. Plain, accurate and cheap — the quiet workhorse for a mid-size tank.
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👍 Pros
- PTC ceramic heating element resists overheating and thermal shock
- Shatter-resistant body — tougher than a plain glass tube
- Accurate adjustable dial; made in Italy with a solid reliability record
- Fully submersible with a clear, grippy temperature dial
👎 Cons
- Simple dial, no digital read-out
- Long body needs vertical space — awkward in a shallow tank
- Dial markings are a rough guide; set by a separate thermometer
Plain, safe and built to last
The Hydor Theo has quietly heated tanks for years, and its appeal is simple: an Italian-made body, a PTC ceramic heating element that self-limits its temperature, and a shatter-resistant shell that is tougher than a plain glass tube. There is no display and no app — just an accurate adjustable dial and a heater that gets on with the job. For a keeper who wants reliability over gadgets, that is exactly the point.
Sizing the 200 W
Hydor pitches the 200 W Theo at roughly 100–200 L, and the honest sweet spot sits at the top of that: a 150–200 L community tank. Use the 1 watt per litre rule of thumb (about 1.5 W/L in a cold room) and, as always with heaters, oversize rather than undersize — a slightly larger heater cycles off sooner and holds a steadier temperature than an undersized one running flat out. On anything much over 200 L, consider running two smaller heaters for redundancy; the full table is on our aquarium heaters hub.
Theo, or something with a screen?
If you want to see the live water temperature and get a drift alert, the Fluval E200 adds a display at the 200 W size. Prefer a shatterproof plastic body with run-dry cut-off? The Aqueon Pro 150W is the tough, cheap alternative. And to hide the heater entirely inside your filter hose, the Hydor ETH inline — from the same maker — does exactly that. Match the heater to the tank on our aquariums page and pair it with the right filter.
The dependable value heater for a mid-size tank. No screen and no frills, just an accurate, well-built Italian heater with a genuinely safer element — hard to beat for the money.
Hydor Theo 200W — frequently asked questions
How many watts do I need?
Budget roughly 1 watt per litre in a normally heated room, and about 1.5 W/L if it is cold. So the 200 W Theo suits a 150–200 L tank. Between sizes, oversize rather than undersize — a larger heater simply cycles off sooner, while an undersized one runs constantly and still struggles on a cold night.
What is a PTC heating element?
PTC (positive temperature coefficient) ceramic self-limits its own temperature — its resistance rises as it heats, so it physically resists running away to a dangerous temperature. Combined with the shatter-resistant body, it makes the Theo a safer bet than a bare-wire glass heater, which is a big part of its long-standing reputation.
Is the dial accurate?
It is accurate and stable once set, but treat the printed dial numbers as a starting point rather than gospel. Set the dial, check the water with a separate thermometer after a few hours, and nudge the dial until the thermometer reads your target. After that it holds temperature reliably.
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