Eheim Jager 100W Aquarium Thermostat Heater
Accurate to within a degree, recalibratable, and with the longest reliability record in the hobby — the default freshwater pick.
- +Holds temperature within a degree
- +Accurate thermostat with a calibratable TruTemp dial
- +Auto shut-off when it runs dry
- +Fully submersible
- +Runs for years
- −Bulky glass tube
- −Suction cups fail over time
- −A minority of owners report the dial needs recalibrating to read true
This is the Eheim Jager 100W, a fully submersible glass thermostat heater with a calibratable TruTemp dial and an auto shut-off that trips if it is run dry. At 100W it suits roughly a 20 to 30 gallon freshwater tank in a normal room. It is UL listed and has the longest reliability record of any heater in the hobby.
- Freshwater tanks around 20 to 30 gallons that want an accurate, long-lived heater
- Keepers who want a dial they can recalibrate rather than trust blindly
- Anyone who values the run-dry auto shut-off and a UL listing
- Nano tanks under about 10 gallons, where 100W is too much and a lower wattage is safer
- Anyone who needs a compact or shatter-resistant heater, since this is a bulky glass tube
- Keepers who will not run a separate thermometer, which every heater needs
We separate what the maker claims from what independent sources confirm, what owners report, and where we are drawing an inference. We do not bench-test products.
- Accuracy stated at plus or minus 0.5C
- Calibratable TruTemp dial
- Auto shut-off when run dry
- UL listed and fully submersible
- It is a glass-tube design, accurate but bulky
- 100W sizing suits roughly a 20 to 30 gallon tank
- The dial can be recalibrated to correct any drift
- Owners praise reliability and heating performance over many years
- A minority report the dial reading a degree or two off until recalibrated
- The recalibratable dial, run-dry shut-off and long track record make it the default freshwater pick
- As with any heater, pair it with an independent thermometer rather than trusting the dial alone
The heater is a one-time cost and typically runs for years. The only ongoing cost is electricity, and a 100W heater cycling in a normally heated room costs only a few dollars a year. There are no consumables; the suction cups are the wear part and are cheap to replace.
- Suction cups weaken over time and the heater can drop against glass or substrate
- The dial can drift, so recalibrate it against a thermometer
- Glass can crack if the heater is lifted out of the water while still hot
- If a thermostat ever sticks, temperature runs away, so run a separate thermometer and consider a controller for valuable livestock
We do not bench-measure temperature accuracy; this synthesises Eheim's stated specification and aggregated owner reports. We give gear guidance only, not livestock or disease advice; route fish-health questions to a veterinarian or an established authority.
Roughly 20 to 30 gallons in a normal room. Undersize it and it struggles in winter; oversize it and it can overheat a small tank.
Yes. Run an independent thermometer alongside any heater and recalibrate the TruTemp dial to it if the reading drifts.
No. Unplug it and let it cool in the water first. Pulling a hot glass heater into the air can crack it.
It works in both, though it is bulky. Many reef keepers prefer a slimmer or controller-driven heater.