Tropical fish usually require water to be about 78 degrees Fahrenheit.
There are species of fish, however, that require 74 - 84 degrees
Fahrenheit. A heater that is thermostatically controlled and the correct
wattage is a must for your fish.
The most basic heater will have an electric element inside of a water-tight
glass tube. When the water temperature drops, the heating coil will
automatically be turned on by the thermostat. There are two types of
thermostats, an internal thermostat is within the glass tube; an external
thermostat will be attached to the outside of the aquarium. Units that are
combined are easier to connect. The type you select is a matter of
preference, but any unit needs to be submerged at all times and clear of
gravel allowing the water to circulate completely.
The correct size is very important; a small heater will run constantly and
soon wear out, requiring replacement. A good rule of thumb is that each
gallon of water should have 5 watts of power. If you have any questions
concerning the wattage you need, inquire where you are purchasing your
heater. Aquariums over 3 feet long are best heated with 2 units to ensure
evenly heated water.
Since water temperature needs to be constant, an aquarium thermometer is
necessary. Attach a floating thermometer to the inside of the tank with
suction pads. There are also thermometers that fasten to the outside of the
tank. If you purchase the liquid crystal type do not put it in direct
sunlight, it will affect the reading. A darker or shaded area of the tank
After you have placed the heater in your tank, start it at a low
temperature reading and slowly advance to the desired temperature, using
your thermometer as you go. When you reach your desired temperature it is
vital to keep it there - quick changes in temperature can not only cause
stress, but can also cause disease or even death.