Once you place the heater in the aquarium it is easy to just forget about it. You should check from time to time to ensure that it is doing the job. Here are a few things you can do: Check the indicator light on the heater. Find out if it is only the light or if the heater has some other malfunction. Use a thermometer or even get your hands wet to check the temperature. First, do the check before turning on the heater and then turn on the heater and give it some time. Now check again. Sometimes the heater automatically changes the temperature. Set the heater at a higher level and then check after some time. One better way to check is by using a torch to watch the water flow around the heater. If it looks blurry, then you know that it is working.