We discussed heating the entire fish room, rather than individual tanks in the section in an earlier design section (Moisture, Heat, and Power). It is much less expensive and more convenient to heat the entire fish room, rather than individual tanks. However, if you do have to heat individual tanks, or have a tank outside your fish room, in this section we will discuss your choices.
There are two basic ways heaters are constructed and two ways their control systems work. There are submersible and non-submersible, and there are analog and digital heaters. Non-submersible heaters are really inconvenient during water changes. If you do more than a ten percent water change, you have to unplug the heaters, or have them all plugged into a power bar so that they can collectively be turned on and off. If you forget to turn the heaters back on, your fish may become chilled. If you forget to turn them off, the tube may become so hot that it will crack.
Guppy tanks are usually small, so in most cases a 25 Watt heater will be sufficient for a 5.5 gallon aquarium. It will also serve a ten gallon aquarium if the room never cools more than 8 to 10F degrees than the tank. A rule of thumb is 2.5 Watts per gallon.
Heaters do fail, and cheap heaters fail sooner and more often. Often they stop generating heat. Sometimes they overheat. If you use individual heaters, you should have a thermometer in a prominent place on the front of your tank, and it should be large enough to call attention to itself. Usually a heater fails because the contacts that turn the heater on or off become stuck open or closed. This is a good reason why you should buy a digital heater, one that does not use contacts to turn it on and off. This is especially true in small tanks which heat up and cool more rapidly than large tanks. Conventional analog heaters are overworked. Also, if you use a heater that is over- or under-rated for the tank, the heater will turn on or off more often to try to stabilize the temperature. Smaller, 25 Watt heaters are sometimes hard to find, and it is over-rated for a 5.5 gallon tank anyway. A digital heater is the safest way to go with 5.5 gallon tanks and smaller.