Updated: Mar 18, 2019
A normal piece of equipment that not every fish tank has (but should) is a thermometer. Most aquarium heaters that are bought today are automatic. They turn on when the water dips below a certain temperature, and turn off when they reach that temp. So what is the point of having an eyesore of a thermometer? Well the answer to that is when something happens to the heater.
Yesterday I noticed that my 45 gallon tank was at a temperature of 84F. This is really high for a community tropical tank and can be harmful to the fish. At first I thought that the thermometer was broken. But after getting a second thermometer, I discovered it was in fact right. What was broken was my heater. I have one of the heaters that was mentioned earler that is supposed to be automatic. I discovered that it never turns off. For some reason the heater just stayed on even when the water was at the correct temperature or even hotter.
I went to bed with the mindset of if the tank was still hot when I woke up, I would get a new heater. Today when I woke up my water was still too hot for my comfort. This led me to going out and buying a new 150 watt heater for my tank. I am happy to report that this heater has proven to keep the temperature at a comfortable 78F for the fish. If I never had a thermometer on my tank, I would have never caught this malfunction.
Having water that is too warm is lethal to fish for a reason that not many would think about it. They don't overheat to death normally, instead they suffocate. Since warm water doesn't retain oxygen as well as cooler water, if the tank is run at a higher temperature, there is less oxygen for the fish. I think what saved me from having any casualties in this was the fact that I keep an airstone running all day to keep oxygen in the tank. I also have a lot of plants that give off oxygen in my tank.
If my heater broke and I didn't have the airstone or plants, I may have not been so lucky with avoiding death in this situation.