I have 40 years of experience raising salt water and freshwater fish. I purchased this heater because Eheim was historically a great name and very high quality. The reviews showed 83% four or five stars. I needed a new 300 watt heater for my 110 gallon cichlid tank and had good luck with Eheim products in the past. I didn't mind paying more for a really critical element in my aquarium set up. Plenty of horror stories about heaters malfunctioning and frying everything in the tank. Right out of the box the heater was bad. No mater what temperature you set, the heater stays on continuously. I couldn't get the heater to turn off with any setting including the lowest. Heater was completely submersed and I even waited overnight to give it another try. Only reason I give it one star is because there is no option for zero stars. I'm hoping that as a very good Amazon customer they will take a return and refund my purchase. They say "You get what you pay for." Apparently no longer the case with Eheim. I guess I should be happy that the heater's thermostat was stuck "on" instead of working initially and then sticking on and frying all my fish. With a critical item like the heater I always tried to buy the best since a product failure would cost far more than a few extra bucks for the best that money can buy. Hopefully my local aquarium retailer will have something to offer that is higher quality!

These are the best I have ever found. No other heaters have given me pinpoint accuracy in keeping my aquariums spot on with no variance in temps. Ignore the dials temp markings...it will lead to baldness from pulling your hair out trying to calibrate this to correspond to the marking on the dial. Guess what? The fish don't read what the dial is set to. Use a thermometer and dial it +/- slowly until desired temp is reached. Do allow it to normalize for 24 hours because if you grab the knob and turn it until the light comes on/off, you will be wasting your time. There is a delay built in...and that is why you need to wait and watch it cycle after you dial. These are really good within 1 degree water temp fluctuations and i have tried many many others over the years with substandard results. Use an accurate thermometer and you will see. Don't throw it into your aquarium hoping to get the perfect dialed in temp out of the box with all of your expensive fish...instead, try throwing it into a 1 gallon water jug with slits cut into the top (Assuming you have the 50 watt model or smaller) so the heater goes down below the line marked on the body and adjust accordingly.

What a long.................long heater. Be sure you have more than 20-inches to spare for this big boy to fit in your aquarium. It will work both vertically or horizontally. Along with its impressive size is its impressive and accurate heating ability. I upgraded from a 55 gallon tank to a 150 gallon tank measuring 30" tall. The poor little heater that was in the 55 gallon tank was only rated for 100 gallons and it tried so hard to keep my fish warm. He chugged along for 4 days while waiting for this big boy to arrive keeping the water at the bottom of the heat chart for tropical fish. Once I plugged the Eheim heater in it took only 2 hours and the tank was a toasty 78.9 degrees.. Perfect for my little swimmers. This heater cycles quickly and a drop of only .02 of a degree will make this bad boy jump into heater action. The amber indicator light glows quite brightly and the only drawback (if there really is one) is that the heater only glows when heating. Once it has reached the desired temperature, it just hangs there in dark silence. Most heaters glow green when the water has reached the proper set temperature but this guy doesn't. If you have a large tank, this is the heater to get. With a 3 year guarantee backing it up and a very reasonable price you can't go wrong. Just remember that he measures a whopping 20" so he needs lots of room.

I have never written a review before but thought this would be great information for anyone that reads it. When this item was purchased In February it worked flawlessly, i set the temperature as i needed it and it was great. 7 months later i had adjusted the temp up 1-2 degrees on the twist knob to accommodate new fish placed in my setup. The unit then never stopped at the set temperature and continued to heat the water up to about 89 degrees (up from 82) when i caught it, it was still on and heating. Half of my fish have died. I have messed around with it a little bit now and it seems that something internal is sticking making the unit worthless. So, just a heads up if you have or get one of these and adjust the temp. Keep a close eye on it.

Listen, I know what your thinking:

Here's a guy who is giving this one star because he's a moron and can't use an aquarium heater correctly. That's typically what I think when I read a poor review on Amazon.

Wrong.

I have been keeping reef aquariums for a long time. Back in the day, Eheim Jäger heaters used to be bullet proof. My the times have changed.

I recently had a catastrophe involving one of these heaters (the 300 watt version). I used this heater, with a Ranco controller, in tandem with a 20 year old Rena top light. I bought the jäger back in July of 2015 when I was doing my current build.

Today, my ground fault kept tripping, and during the investigation I found that the wires had melted from the heater body and water had filled the heater tube. My fish were near shocked to death and thankfully have recovered, but all of them were laying sideways on the ground. Thank god none of my kids put their hands in the sump or the tank or they would have received a pretty good jolt.

See the pics below for details. I would never ever in 100 years buy another one of these heaters. Never.