Reviews ~ Hydor ETH 300 In-Line Heater

Hydor ETH 300 In-Line HeaterMy planted aquarium layouts are pieces of art in my home. I choose plants, rocks, wood, and fish carefully to ensure that they all work together to create a layout that is interesting, natural and pleasing. When I enjoy my planted aquariums and when others take time to view my work, I want the focus to be on my efforts and the beauty of the design, not on the necessary hardware needed to keep planted aquariums.

I try to minimize the amount of hardware in the tank. The Hydor ETH 300 In-Line Heater solves this problem for me. The 300 watt heater connects to the outflow of a canister filter, evenly distributing heat throughout the aquarium, while staying neatly hidden in my aquarium cabinet.


The Hydor ETH 300 In-Line Heater is rated at 300 watts. Conventional wisdom suggests that 300 watts would be sufficient for up to a 60 gallon aquarium. I have used this wisdom for years, but it is very conservative for an in-line heater. The circulation through this heater allows it to work much more efficiently. I have used a single ETH on a 90 gallon aquarium in rooms that drop to the mid-60's in the winter, and the heater had no problem maintaining 78 degrees.

Setting the Temperature

Setting the temperature on the heater is simple. No more wet fingers!! Setting the working temperature is done using the red bezel on the outside of the heater. The temperature settings are clearly marked, and once it is set, I have found the unit to be very consistent. A red LED lets me know when the heater is on.


The Hydor ETH In-Line Heater is constructed of durable plastic. Inside is a glass tube through which the water passes. Wrapped around the glass tube is a conductive heating element that heats the water as it flows through the tube. Each end of the glass tube is attached to a connector and sealed with a rubber gasket. The connectors are designed to clamp snuggly on the filter tubing, which in the case of the ETH 300 is 5/8" tubing.

Hydor ETH 300 In-Line Heater

The Hydor ETH 300 In-Line Heater disassembled


Hydor ETH 300 In-Line Heater

5/8" Connectors

When Things Go Wrong

The heater has a safeguard to protect it from overheating. This safeguard is designed to protect the heater in the case of loss of water flow through the heating element. For 5 years, this failsafe has for me.


One day, I opened my aquarium cabinet to see a small puddle of water gathering under the heater. I can not be sure of the cause, but what I am sure of is that the glass tube cracked causing the heater to leak. I quickly unplugged the unit and replaced it.

If I had to guess, I would say that the power went out sometime during the day. When it came back on, the filter failed to move water and the heater failsafe was unable to turn off the heating sleeve. Thus, the glass was heated to a high tempature. It is also probable that the filter finally started moving water and the cool water flowing through the very hot glass tube caused the crack.

Below are pictures of the broken heater.


Hydor ETH 300 In-Line Heater

Here is what the sleeve looked like after the crack


Hydor ETH 300 In-Line Heater

You can see the crack just under the rubber gasket


Hydor ETH 300 In-Line Heater

Removing the gasket shows a better picture of the crack


Hydor ETH 300 In-Line Heater

This is what is left of the heating sleeve after removing it from the glass tube


Hydor ETH 300 In-Line Heater

The epicenter of the crack is visible in this picture of the glass tube


I would like to reiterate that I have had a few of these heaters over the years and they have all worked flawlessly. That being said, it further proves the wisdom of regular inspection of all hardware that is used in aquariums.




Numbers – Rated 1-5 (5 is the best)

Price: 5
Construction: 5
Effectiveness: 5
Value: 5


Heating an aquarium more efficiently than a standard heater, while getting hardware out of the water column is a winning combination. Adding temperature consistency, rugged construction, and a reasonable price makes this a must-have item.



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