Any heater can get stuck in the "on" position, especially but not exclusively the ones with bimetal control units (which is what most heaters, including Jaegers, use). With 21 tanks and 44 years of fishkeeping experience, I have seen it happen all too many times. No matter who is the manufacturer, or what style or model you choose; they all can and do malfunction.
So the best advice out there is not to overheat your tanks, and to split the duty between two heaters so that one sticking in the "on" positon won't boil your fish. If you have a 20g tank and your ambient room temperature doesn't drop below the mid-60's, you should be able to get away with (2) 25W heaters. Even if one just flat out fails and doesn't generate any heat at all, your tank shouldn't drop to dangerously low temps, as long as the other is still working.
So the best advice out there is not to overheat your tanks, and to split the duty between two heaters so that one sticking in the "on" positon won't boil your fish. If you have a 20g tank and your ambient room temperature doesn't drop below the mid-60's, you should be able to get away with (2) 25W heaters. Even if one just flat out fails and doesn't generate any heat at all, your tank shouldn't drop to dangerously low temps, as long as the other is still working.
