As colder weather is moving in, it's a good idea to check your heaters. This piece of equipment has probably killed more animals in an aquarium that any other piece there is. We get calls all the time all of "my fish are dead in my tank water almost to the boiling point".
As a general rule, we use 5W per gallon to keep the water at an optimum temperature, but what we do is we usually divide the Watts by 2 and pick dual heaters. We set them a few degrees apart, that way if there's ever a failure your water should not get too warm. Plus 2 heaters have a tendency to make water a little bit more safe. The 2nd one will only kick on if it set a few degrees colder on so you'll actually expand the life of your heaters.
When placing your heater in your aquarium, make sure you're getting lots of water flow across the heating element. This will make sure all the water is heated more evenly if your tank or your filter is made of acrylic. Also, make sure you isolate it, so it will not melt any of the plastic. We put our heaters inside of the overflow box and the probe outside. This allows for good amount of contact time before the heater will shut back off. If it breaks, you don't have glass in the aquarium or in the Filter Box.
We usually try and go with heaters that are made of titanium. They seem to be able to take a little bit more abuse glass heaters if they accidentally get dry will shatter When cold water contacts them. Titanium ones just heat up and some of them are made with a wax plug inside of them so if they overheat they will blow the fuse instead of shattering in the tank or overheating. It's also a good idea to replace heaters every few years, and if you live in a place where the weather changes quite a bit in the temperature fluctuates constantly probably a good idea to do it every year