Many people underestimate how much of a difference a heater can make in an aquarium. It is not that some heat better than others, almost any of them can keep the temperature right around where you set it. It is when things go wrong that they make a massive difference.
Most types of heaters fail. Working in service for so long we see the problems on a larger scale. It seems that every year whatever heater we were using would start causing problems for a few of our customers or clients. Usually the problem is that they get stuck on. Even the best of the high quality heaters can do this. With the number of aquariums most aquarists keep it may not ever happen or it only happen once or twice. Working in service we see it on a larger scale when we see service clients and retail customers have issues.
Heaters should be the minimum power needed to heat the tank. This is usually much less than the packaging will lead you to believe. In most cases the ratings on the boxes are based on raising the temperature 15F or more over room temp. Most tanks will not need even close to this amount of power. Unless you keep your house exceptionally cool in the winter you shouldn’t need to raise the temperature more than about 4-8F over room temp. If you have discus they may be an exception. So if the usual rule is about 5 watts per gallon of heater, you likely need only about 2 watts per gallon. Also keep in mind that larger tanks have a more stable temperature because of the large volume of water. In these situations you may need even less wattage.
The ideal setup is to have two heaters that are each half the wattage of what you actually need. So if you need 200 watts total getting two 100 watt heaters is a much safer option. This means that if one of the heaters ever gets stuck on the other one can simply stay off. This can result in a very minimal, if any, actual increase in tank temperature.
It may sound like a waste of money at first, but regularly replacing the heaters will almost always prevent any issues. If you prevent the problem before the problem even exists then you win. Replacing the heaters every year has become the standard for my service clients. It simply makes too much sense to not do. The amount of damage that can be done by one heater getting stuck on greatly outweighs the minor cost of a new heater every year.
I can’t tell you how many clients and customers have had heaters nuke their tank. I have had it happen to me and countless customers and clients have had the same issue. Sometimes you catch it, sometimes everything dies. Losing an entire tank’s worth of fish you have been raising for years isn’t worth saving $60 or so on heaters. Just replace them regularly.
As for types of heaters, I will never use glass again. They all can end up leaking, even the best brands like Eheim. I only use the black thermal plastic heaters. My second choice is the Aqueon Pro, but my top choice is the Cobalt Neo-Therm. They are shatterproof black thermal plastic so they blend in very well. They have an LED dial that indicates the set temperature and the actual temperature. The Aqueon Pro is also shatterproof black thermal plastic and does have an LED indicator light, but instead of showing what the temperature is, it is just red if heating and green if not. The extra accuracy of the Cobalt Neo-Therms is worth the slight price difference.