The term fish tank heater is somewhat of a misnomer. Most fish tank heaters are not in fact heaters at all. They're more of an air conditioning system for the tank. Rather than simply heating the water up, they carefully monitor and regulate the temperature of water.
You might wonder why such a thing would ever be important. Why can't you just purchase a part of pants for your fish. They sell pants for dogs, don't they? Why not fish?
Unfortunately, fish pants haven't yet hit the market. Until they do, aquarists (those who build aquariums for a hobby) will have to use a fish tank heater to condition their tank to fit the right temperatures.
For instance, if an aquarist was working on a tropical tank, he would want to set his fish tank heather to a setting that was above room temperature (assuming he doesn't live in the tropics).
In contrast, if an aquarist was working on a cold, salt water tank, he might want to set his fish tank heater below the room temperature.
One of the more popular models of fish tank heather is the “visi-therm.” This model looks like a thick, metal thermometer. It can attach to the top of the fish tank; and it operates by sucking in water and heating it. It is double thick and is water-proof. It sells at most PetSmart stores for around $15.
If you're looking for a cheaper model, you could always go with the Therma Flow PC Plus Heater, which sells for as little as $7. On the other hand, if you have a bigger tank or simply want a higher-end product, you could consider one of Visi-Therms more expensive models.
These models will regulate temperature considerably faster, but at a price—most of these fish tank heaters start at around $20.
More Aquarium and Fish Tank Info:
Aquarium & Fish Supplies (home)
Cleaning a Fish Tank
Gravel for Fish Tanks and Aquariums
Fish Tank Maintenance
Fish Tank Pumps
Aquarium Screen Savers
Aquarium - Fish Tank Stands