We've all done it a few times. You're heating a bucket for a water change and just quickly swap the heater from the tank to your bucket. Sure the warnings say to keep the heaters under water and just being exposed for a second or two can have no dangers right? Owners asking the question couldn't be more wrong.
In the early stages of my aquarium keeping I frequently did this process. Water changing several tanks was a hassle and I wasn't about to waste time going to unplug the heater every time I had to move it around. As I'm filling one of the tanks and remove the heater, ready to place it in an awaiting bucket I hear a loud popping sound and feel a pain in my right leg. The glass rod of the heater had exploded, propelling the glass into the side of my leg. Fortunately I had multiple layers of clothing to soften the blow, leaving me with only a few bruises and burning my pants. The cost of the pants and cheap heater combined far outweighed the best aquarium heater I could find to replace the old one. If I had been in shorts or even a more thin pair of pants The medical bills would have cost me more than the entire tank! Luckily my leg only hurt for a week or so and healed on its own.
After this experience I was livid. I thought the company had sent me a bad heater and nearly caused me injury. After a few quick searches and phone calls I found out this is not a rare occurrence. Taking a glass aquarium heater out of water for even a second can cause shattered glass. Furthermore putting the heater in different temperature water can have the same effect.
As tedious as it may seem, the right way to move heaters is to turn them off before removing from water and allowing them a few minutes to cool down. While changing from 120 degrees to 60 degrees will often cause a destroyed heater, falling from 80 to 60 degrees will not cause the heater any harm.
This includes taking the aquarium heater out of water. The air in your home is a quick way to cool the heater, however it will do the job far too quickly. Leave the heater in your aquarium for at least 3 minutes after unplugging it to safely remove the heater. If you will be placing the heater in new, cold water than allow the heater to cool further outside of the tank before submerging into the cold water. Follow these steps and you can avoid the injury I had to learn from.