I bought an Aqueon 15 gallon set-up with an automatic heater. According to two different thermometers, the water is not in the "ideal" green area, and is only about 68-70 degrees, which I think is not warm enough for my angelfish. My question is should I have the heater completely submerged (as in part of the cord in the water)? Seems unsafe to have an electrical cord in the water, but the instructions suggest that the heater can also be positioned horizontally, which would obviously have the cord in the water. Instructions give a minimum water level, but not a maximum. Thanks.