i got this 100w heater, 4got the brand but i got it off ebay.
so i use it inside my 5.5 gallon with 4 inch of water(breeding betta)
i set it at 82 but went i came home from work i check the temp and it was at 77. i was like WTH since it was set at 82 it should work since the tank is only 4inch of water. so i turn it up to 86 but it still at 77. so i turn it up more to 88-90F. later i check the water it was at 82 F.
whats going on what this stupid heater? the heater not doing it job.
Looks like a ery cheap Chinese made heater. Most heater scales are off by a bit. The cheap ones, who knows? That's why you need a thermometer to check. Cheap heaters normally fail in the on position so they can cook a tank. Newer quality heaters fail in the off position.
Part of your problem is that you need 25w or 50w maximum heater for that little water, unless the room ambient temperature is like 30 degrees colder than the tank.