Hydor Theo Shatterproof Aquarium Heater, 150-Watt & Ideal For 23 To 40 Gallon Fish Tanks, 12-Inches Long

I really like this heater, even though it is costly for its wattage.Small tanks, typically used for Bettas, always have the issue of heating them and due to their small size most heaters will not fit. Well, this small 25W heater works perfectly in my 5.5 gallon aquarium easily fitting vertically with plenty of space to spare above and below. The thermostat on top is labeled in temperature, versus just a + and - or arbitrary scale (like 1 to 10). The temperature may not match the dial right out of the box, but really that's not a big deal. If you need it a degree warmer, you turn the dial up 1 degree. This is assuming you are not exceeding the capabilities of the heater, which I explain in more detail below.I'd like to point out that this is a very low wattage heater. So while it works fine on my 5.5g aquarium you should know that it will only do fine if your room temperature is somewhat close to your desired tank temperature. If you keep your tank at 78 degrees, but in the winter your house is at 65 degrees this heater will not be able to make up the 13 degree difference. Small tanks loose heat fast, and this heater at just 25W will not be able to keep up with that.Because of this some people will mistakenly say "It can't heat my tank even when I turn it up to high". As I mentioned earlier, these have a thermostat so they work only on temperature. If it is set for 78 and the tank is only getting up to 76, it is doing its best to heat the tank to the set 78. Turning it up to 85 degrees does not change that. The heater will not turn on, and stay on 24/7 and the reason is so that it doesn't super heat itself.How low your house can be for this to work will depend greatly on your specific tank dimensions, what's in it, and your filter flow. Because of that I can't give you some specific number, but as a ball park in a 5 gallon aqiarum I doubt you could go under 70 degrees in your house and maintain 78 degrees in the tank.The heater has a fill line on it, this is a minimum fill line, which means the water must be at least up to the line for the heater to work without breaking. That line is not a maximum, you can fully submerge this heater with no issue.

In late March 2012, I was unaware that my 2 old aquarium heaters had stopped working. I happened to notice that the water temperature was not even registering on my digital temperature strip on my aquarium front glass. This meant that the water temperature was somewhere in the 60's.Upon inspection of the 2 heaters, the glass in both of my old aquarium heaters had shattered, evidently when the water level had gotten too low causing the glass tube to break and they stopped working.I lost a few fish due to the water being too cold.I immediately searched Amazon for a brand of heaters that would be unaffected if the water level ever got too low.I found these Hydor brand heaters listed on Amazon, their advertisement stated that they would not be affected by low water levels. Unlike most other brands, they would work even if they were not fully immersed in water.On April 4, 2012, I purchased 2 of these Hydor 400 watt heaters from Amazon for my 135 gallon aquarium. For one year, they kept my tank at exactly 74 degrees year around. The tank's water temperature never varied more or less than 1 degree from the 74 degrees that I had them set for. I absolutely loved them. They were the best heaters that I have ever owned in my 40+ years of owning a large fish aquarium.Now, for the rest of the story as to why I am now unhappy with them.Our fish aquarium is located in our basement which stays at 60 degrees year around so good dependable heaters are a must. Whether it is winter or summer, our basement stays between 59 and 60 degrees constantly. 3 different furnace companies have yet been able to solve this problem but it's a great place to be when it reaches 100 degrees outside.Exactly one year and one week after installing these heaters, I decided it was time to do a total tank clean up and a 30% water change. I unplugged and removed both of the heaters and placed them on my workbench which was close by. This was the very first time that I have ever unplugged the heaters since April 2012.After I had finished with my cleanup and water change, I reinserted and replugged both heaters back in. I did not touch the settings as they were both still set at 74 degrees. The next morning I noticed that my aquarium water temperature was 82 degrees. Both heaters were running non-stop, thus overheating the water. I immediately unplugged them and let the water cool back down. After the water had cooled and reached 74 degrees once again, I plugged the heaters back in again, making double sure that they were set at 74 degrees. Both heaters came on, they were both on constantly as the water temperature rose to 78 degrees within a few hours. I adjusted the heaters to their lowest possible temperature settings and tried them once again but the heaters were still on constantly and the water temperature was once again rising.Over the next several days, I tried numerous times to manually shut them down to lowest possible temperature setting, unplugging and replugging them, moving the control knobs back and forth, but they would still heat the tank to over 80 degrees. Up and down water temperature changes are extremely hard on tropical fish.It's like the heaters were stuck wide open to the maximum temperature and the shut off sensors were not functioning. I found the only way to shut them off is to unplug them.I don't know if unplugging and removing them from the tank may have caused them to stop working correctly. Maybe I should have waited an hour or so before removing them from the tank. At any rate, I cannot use them in my aquarium any more.I have ordered two new 300 watt heaters of a different brand from Amazon. Amazon's 2 day delivery is a life saver as the heaters will be here tomorrow. I will keep the Hydors just in case of an emergency. I believe these heaters have a 2 year warranty.

I purchased this heater 2 years ago for a 2.6 gallon Fluval aquarium. It worked well and kept the water the same temperature that I set it to for a few months. I stopped looking at the thermometer in my tank as frequently because it was always the correct temperature. You absolutely must keep an eye on the thermometer multiple times a day with this heater, especially the longer you use it. Do not trust it. This heater literally melted off the fins of my betta fish while we were on vacation and didn’t leave our house’s air conditioning on. The house didn’t reach over 78 degrees, but the heater kept turning on and heating even though it wasn’t needed as the water matched the temperature of the house. I have observed the variation in temperature very often now that I work from home, and it is shocking how inaccurate and “stuck” it is. I have the heater on its absolute lowest setting and my tank still heats up to 85-90 degrees sporadically, even though we don’t let our house get over 75 degrees when we are home. I’ve gotten to where I unplug it at night because I don’t want to wake up to another stressed fish with melted fins. It’s so frustrating because our house is much cooler at night, so my fish has to be cold due to an untrustworthy heater.