Eheim – 125 Watt Jäger TruTemp Submersible Aquarium Heater

Precise, German-made heating for mid-size aquariums of about 40 to 53 gallons. The Eheim Jäger TruTemp 125W (sold internationally as the EHEIM thermocontrol 125, model 3615) is a fully submersible glass heater that holds your chosen temperature anywhere from 65 to 93°F. Its TruTemp recalibration ring lets you match the dial to your thermometer, and automatic dry-run protection shuts it off if the water level ever drops too low. Rated for freshwater and marine tanks and backed by Eheim's 3-year guarantee.

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Why Keepers Trust the Jäger

• TruTemp recalibration. If the dial and your thermometer disagree, reset the red marking arrow to the temperature you measured and the scale reads true again.
• Wide, accurate range. Set anywhere from 65 to 93°F (18 to 34°C). Eheim rates the control accuracy at ±0.5°C (about ±0.9°F).
• Automatic dry-run shut-off. Eheim's Thermo Safety Control switches the heater off if the water level falls too low, then resets once it cools.
• Laboratory glass jacket. The outer glass sleeve increases the heating surface for even heat output.
• Fresh or salt. Fully submersible and rated for both, with a control lamp that shows when it is heating.
• 3-year Eheim guarantee. Made in Germany.

Specifications

Setup and Calibration

• Mount it in the flow. Clip the suction holder on away from the heating zone and control lamp, then mount the heater vertically in a spot with strong water movement, with the water above the MIN mark.
• Let it acclimate. Leave the heater in the tank water, unplugged, for 20 to 30 minutes before switching it on for the first time.
• Set your temperature. Turn the blue adjusting ring back to the 65°F stop, then dial up to your target.
• Recalibrate if needed. After 1 to 2 days, compare a good thermometer with the dial and use the TruTemp ring as described in the FAQ below.
• Unplug before you lift it out. Let the heater cool in the water for at least 30 minutes before removing it, and give the cord a drip loop below the outlet.

Complete the Setup

• Squeaky's Digital Thermometer to check your water temperature and calibrate the TruTemp ring
• Eheim Jäger TruTemp 100W for tanks of about 26 to 40 gallons
• Eheim Jäger TruTemp 200W for tanks of about 79 to 106 gallons

FAQ

What size tank is the 125W for?
Eheim rates it for aquariums of about 150 to 200 liters, roughly 40 to 53 US gallons. In a cool room or near the top of that range, many keepers add a second heater or step up a size.

My thermometer and the dial don't match. Is it broken?
Not at all. That is what the TruTemp ring is for. Measure the water, unplug the heater, turn the red arrow to the temperature you measured, then reset the blue ring to your target.

What happens if the water level drops?
The built-in thermal circuit breaker switches the heater off when it is out of the water or the level falls too low, and it turns back on automatically after cooling. Keep the water above the MIN mark, and always unplug the heater and let it cool before lifting it out.

Can I use it in a reef tank?
Yes. Eheim rates the heater for both fresh and marine water, so it works in freshwater, planted, and saltwater systems of the matching size.