Ok. So I got a heater for my 5.5g guppy tank. It was an adjustable one. So I put it in the water, and waited 20 minutes before I plugged it in. Then the next day, the temp was way higher than I wanted. It was like, 98 degrees Fahrenheit (I checked with two different thermometers to make sure). So I lowered the knob on the heater. The next day, it was still the same. So I lowered the knob all the way. The next day, it was still at like, 90 degrees fahrenheit. So I got a new one that works fine, but what was wrong with my other heater? ???