I have a marineland visitherm 100 watt submersible heater in my 20 gal high tank. It is placed in the corner of the tank behind some plants. It was set at 76 degrees and the water temp was around that as well. Recently the water temp has dropped to 72 and I have to set the heater to 85 to get the tank temp up to 75. I am assuming that something is wrong with the heater but was wondering if I could be doing something wrong (i.e. wrong placement of heater). Any thoughts are appreciated. Thanks.