Freshwater

Submersible heaters
As their name suggests, you completely submerse these in the aquarium
water. Ideally, you should place them in a diagonal line across the rear piece
of glass, so that heat flows evenly throughout the tank.
A submersible heater has a watertight glass tube containing an electrical element
wound around a ceramic core. A small red or orange light lets you know
whether the heater is on or off. At the top of the heater is a temperature adjustment
valve you use to raise or lower the heat. Submersible heaters usually
have an internal thermometer that you can see through the glass tube. You
simply rotate the adjustment valve so that the temperature line moves to the
temperature you want to set for your species Other models have temperature
lines (similar to markings on a measuring cup but in degrees) built in with the
control knob. The heater automatically maintains the temperature you choose.
Make absolutely sure that the heater is a submersible model before you put it
completely underwater!
Non-submersible heaters
Chapter 6: Equipment and Other Technical Stuff
Non-submersible heaters hang on the outside aquarium frame, with the glass
tube hanging in the water. The adjustment valve sits above the water line.
This type of heater generally does not have an internal thermometer. You
make adjustments in small increments by turning the valve and repeatedly
checking the temperature. This is the older style of heater.
How to heat your aquarium
The safest arrangement is to keep two heaters operating at the same time
just in case one unit happens to fail. If one does fail, disaster can be adverted
by the second heater, which can keep the water temperature from dropping
while you replace the failed unit.
The rule for determining which size heater your aquarium needs is to allow 5
watts of heater per gallon. For example, a 50-gallon aquarium needs at least a
250-watt heater.
Never remove a heater from the aquarium without letting the whole system
sit for at least 20 minutes after unplugging the heater! Likewise, never turn a
heater on for the first time in an aquarium until after you let it sit in the water
for at least 20 minutes!
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