Heaters

Heaters are electrical units used to maintain a water temperature above a preset minimum.  The easiest and safest heaters to use are completely submersible inside the aquarium.  These work most efficiently if placed horizontally in the aquarium as opposed to vertically.  When placed vertically, the hot water usually rises and immediately warms the thermostat which leads to frequent on-off switching that can wear out the thermostat.  When horizontal, the hot water rises and circulates throughout the tank before hitting the thermostat.  This is much more efficient.

Hanging heaters attach to the rim of a tank, but can crack easily if they come on while exposed to air after some evaporation or during a water change.

Some people have tried heaters that sit under the tank, but these can lead to uneven heat of the aquarium material and weaken it or waste energy heating the air outside the tank or the sides of the tank before the inside.

As a general rule of thumb, you should have 2-3 watts of heating power per gallon in an acrylic tank or 5-7 watts of heating power per gallon in a glass aquarium.