For the most part, high-quality aquarium heaters that are installed correctly are not dangerous. However, even in the best scenarios, problems can occur. Here’s what to look out for.
First, make sure you purchase a heater with enough wattage to heat your tank. Larger tanks will need heaters with more wattage or may require that you install multiple heaters. If your heater isn’t powerful enough for your tank, it won’t be able to heat the tank adequately, which could stress or harm your fish.
Next, make sure you know the ideal temperature for your aquarium. Different fish prefer different temperatures. For example, tropical fish do best in water that is 75 to 80 degrees Fahrenheit. If you set your heater too high or too low, your fish could suffer.
Another big danger is installing your heater so that it touches the glass or the gravel in your tank. This can create a temperature gradient which adds tension to the glass. It could result in the glass of your tank shattering.
Finally, an aquarium heater without a heater guard represents a danger to your fish. Though unlikely, it’s possible that your fish could get stuck between the heater and the glass, which could result in burns on your fish. The best way to avoid this danger is to purchase an aquarium heater with a built-in guard or to purchase and install a separate heater guard around your heater.