Agreed, for 55 and smaller. It is a good idea to have an extra laying around like you just found out. I will say I think most heaters today are **** compared to the ones being made in the 80's and 90's. I still have a few of those non- submersible ones that would screw onto your tank and use them on some of the smaller tanks. Ebo- Jager was the best submersible one, lasted forever and unbreakable. But they got bought out by Eheim and weren't the same. I had two 300 watt eheim on my 265 for the past 12 years and they worked pretty good for a number of years. I slacked on maintenance for a few years when my kids were younger and lost some interest in the hobby but kept the tank running. A couple months back I did a good water change and my interest peaked again. I had one of those stick on the outside thermometers but that hadn't worked in years so I bought a couple of the ones with probes from Amazon. The temperature was 76. I used to keep it at 78 religiously. So I played with both heaters and was able to consistently get the temp to stay at 79.2. Then one day I came home and the tank was at 82 and both heaters were still on so I was like wtf. After that I couldn't get either heater to stabilize so I wound up buying an 800 watt titanium heater with a temperature controller. Best purchase i have ever made. The tank stays at 78.6 with nothing for me to worry about. There is an alarm you can set that will alert you if the tank gets too hot. Mine is set to alert me if it gets to 82 but I haven't seen the temp go up past 78.6 or dip below 78.4. For bigger tanks, I would never use a regular heater again. Just went back to the website I bought it from and they make them as small as 50 watt so you can get them for smaller tanks. The prices are on par with the cost of an Eheim, you just have the added cost of the temperature controller which cost me $65. Well worth it imo, especially since I was replacing two heaters. One less piece of equipment in the tank and way more stability. Sorry for the rant but I'm a little bored right now haha.
Edit: For the record, I didn't use a curse word above. I guess you can't say C R A P on here haha