a while ago i bought a 100 watt topfin heater for my 20 gallon cichlid tank and it gets way to hot. i have the temp knob turned to the lowest setting and it heats the tank to about 90 degrees ferenheight. This heater is rated for 20 gallons so i dont think 100 watts is too much. Does it have a faulty thermostat? is there anyway i can fix this?