Review on Submersible Energy Efficient Controller Thermostat Thermometer by Mia Parks

I have a 5 gallon top fin betta aquarium that currently has one suckling fish (the real name always eludes me) and one medium sized black and white freshwater fish ( his name is Mu-Kuh). I'm not a big fish farmer. The water constantly falls below the "minimum water line" at the filter and at the old dead heater (shocker). I'm having trouble finding a suitable, inexpensive replacement heater with constant reviews that don't mention "cook alive". Initially, the tank had 74 degrees (without a working heater). The temperature steadily rose to 80 degrees for about 12 hours, after which I freaked out and re-read the box. Did I install correctly? Should the device NOT be completely submerged in water? Can Mu Cow survive?!?!Turns out 80 degrees is quite within reach. Moo Cow and Sucky Fish are 6 degrees happier!