Three basic types of aquarium heaters exist: clip-on, submersible, and electronic.
Clip on heaters represent an economical option for providing stable temperature and are designed to be fastened to the aquarium frame. It is important to respect the indicated water line. This will ensure proper functioning of the thermostat.
Submersible heaters offer greater flexibility in terms of application and are available in easy to hide smaller diameters, such as the Hagen thermal compact and preset range.
Electronic heater employs a technologically advanced method of temperature control and eliminated mechanical technologies found in conventional heaters.
There are several ways of maintaining suitable temperatures for the tropical fish. If it is placed in a room which is warm at all times, it may be possible to dispense with special heating altogether if the temperature falls no lower than 68 degrees in the water. Aquarists with many tanks in fish-rooms or fish-houses usually favor general heating of this kind, which is cheaper than individual heating and has the advantage of keeping the aquarist warm as well as his fishes. A vented gas or oil heater is best if central heat is required.
In the average home the single tank will need to be given special heating of its own. The usual heaters are resistance wire wound on ceramic forms and enclosed in glass tubes, sometimes filled with fine sand to conduct the heat faster to the wall of the tube and into the water. These are inexpensive but the cheaper kinds often have a short life. The better ones are more strongly made and have heatproof (Pyrex) glass tubes.
To be effective, the capacity of the heater must be such as to prevent a temperature drop below 65 degrees even in severest weather. This means that the heater must be arranged with an automatic switch to turn the electricity on and off as needed. This switch is called a "thermostat". Most aquarium thermostats consist of a bar of metal (called bimetal), really two different metals fused together, which bends when heated. The bend is always in the same direction and the same amount with the same heat increase. This movement is used to open the electrical contact points.
Construction features vary with price and quality. The oldest and simplest consists of the thermostat enclosed in a glass tube which is suspended in the water. It is connected to the heater which is similarly suspended in the water.
Combinations in which the thermostat and heater are enclosed in the same glass tube are available also, and some are good. These units use heavy bi-metal bars and depend on over-heating in order to operate because the thermostat and heating element are so close together. The contact points on the heavy bi-metal bar usually corrode eventually and stick together, causing your fish to be cooked by over-heating. These units, of whatever construction, are not very attractive in the aquarium, and certainly do not add anything to its value as a "living picture".
A recent development is an extremely sensitive thermostat which is installed on the outside of the aquarium, against the glass. It responds to both the inside and the outside temperatures. This prevents over-heating should the outside temperature rise, and prevents chilling should the outside temperature drop. Thus extremely close temperature control is possible, as close as two degrees F. variation. This is desirable in breeding and raising small fish, but is generally not necessary. A temperature range of 10°F. is usually considered satisfactory.
This outside thermostat is equipped with a very small permanent magnet that causes the contact points to snap sharply together when switching on, and causes an equally sharp snap-off when switching off, thus preventing "arcing" of the points, the cause of corroding. This fast make-and-break eliminates the need of a condenser, another frequent cause of thermostat failure.
The thermostat may be used with the regular suspended heater, or with a submersible heater lying on the sand or partially concealed. The heater size should be from 3 watts per gallon for protected locations to not more than 5 watts per gallon for more exposed aquariums.
So, having decided where the aquarium will go, and how it is to be illuminated and heated, purchase just the tank and necessary apparatus for these purposes. The materials for furnishing the tank will be described in the next chapters.