In my 30 gal. tank I have a completely submersible visa-therm heater. What is the best placement for it? Right now, I have it along the back wall, vertically, with the knob sticking out of the water, so I could adjust it if needed. Since the temperature is now stable, I'd figure I might want to move it into a more hidden location. Are there any problems with putting the visatherm horizontally, along the back wall, about 4 inches from the substrate? Will it still heat the water accurately? Thanks in advance.