It is heart-wrenching to lose everything in your tank from something like a heater going haywire. You may walk by the tank one morning to find everything laying belly-up from a stuck thermostat leaving your tank at 95 F, and your response to this "everything Is dead" scenario will no doubt be extreme sadness. There are some simple things to do which could help prevent this disaster from happening to you.
Don't Scrimp on a Heater
Consider an aquarium heater to be a critical piece of equipment, just as you would a filter or other piece of equipment that is essential to the livelihood of your tank inhabitants. Purchase a top quality unit. Too often the heater is thought of as a simple and inexpensive device, and one tends to cheap out when buying one. What costs more, losing everything in your tank, or spending a little more money on a better heater to start with?
Verify Temps With a Thermometer
Heater temperatures should be confirmed by using a thermometer. From under $2 for a basic thermometer, up to $40 or more for a quality unit with automatic alerts and other features, this is a simple piece of equipment that should be included in every aquarium.
Consider investing in a heater/chiller temperature controller, or another type of device that automatically monitors, controls, and alerts you to equipment and aquarium functions.
Follow Installation and Usage Directions
Pay close attention to the instructions to make sure the heater is not only properly set up but used correctly as well. Some are susceptible to salt creep or burn out. Make sure you have all the proper drip loops or other particular equipment the manual indicates.
Maintain a Regular Cleaning Schedule
Clean the heater and any other components regularly, particularly to keep it free of salt creep, which can leave the unit or control box caked in salt in only a few days. Salt creep is the white buildup of minerals from the water. It is not simply table salt (sodium chloride) but also may be calcium and other minerals. Once salt creep develops, it can take a lot of work to remove and won't simply be washed away with fresh water.
Unplug the Heater During Water Changes
To protect from overheating that can cause damage, always unplug the heater when doing aquarium water changes or removing it from the water. Maintain the level of the water in the tank to keep any particular type of heater submersed properly as instructed.
Avoid DIY Heater Repair
Don't try to repair or use a broken heater. You might be handy, but there is a lot at stake. Discard it and get a new one. In the end, a quality heater (or two) will cost you much less than one (or more) of the fish in your tank.
Keep a Backup
It might be worthwhile to buy a spare heater to have on hand. This can reduce the temptation to limp along with one that isn't functioning well. It is a cheap investment that will save you money and a lot of disappointment in the long run.