Finding the right heater for your fish tank can be tricky. There are a lot of options and different types of heater's out there so it can be hard to know what is the best choice. Here is some information that can help.
There are three main type of heaters you can use for a tropical fish tank:
The most common submersible heaters,
external under tank heaters
Submersible heaters are basically any type of heater that is submersible. These are probably the most common heater, and one of the easiest to use and the maintain a pretty stable water temperature.
Under- gravel heaters are just like the names sounds. They are any heater that is meant to be placed under the gravel. Generally they are mostly used for small tanks that don't have much extra room for a submersible heater.
External under-tank heaters are not commonly used for fish tanks. They are not very practical for fish tanks and aquariums. Generally these heaters are used for reptiles, amphibians, insects, and land hermit crab tanks, not fish tanks.
Most fish-owners seem to prefer submersible heaters. They are easy to find, with all kinds of different features. You can get pre-set ones set already to an appropriate temperature setting usually 78 degrees which is good for most tropical fish tanks. You can also buy ones where you can adjust the settings to whatever temperature you need. This kind is good if you live somewhere that the temperature fluctuates quite a bit. One thing about submersible heaters that you must be careful about is that some of them are not completely submersible.
Under-gravel heaters like I said before generally tend to be used in smaller tanks. Most of these just raise the temperature a few degrees. There is usually no adjustable control. This is fine if you live somewhere that the temperature is constant. If you live somewhere that may get very cold, just raising the temp a few degrees will not be enough.
I would not recommend using external-under tank heaters. They generally do not have adjustable temp. control, and do not heat fish tanks well. They are better used for pets such as reptiles.
When buying any heater you need to do a few very important things:
Make sure it is meant for your size of fish tank. If you have a thirty gallon tank, you don't want to put something like a heater meant for five gallons in it, because it will not be able to properly heat the tank and your fish will be cold and possibly die. You also don't want to put a heater meant for a large tank into a small one because they you will basically fry the fish. So be sure to read labels!
Make sure it will fit in the tank! This usually isn't a problem for large tanks, but for smaller tanks under ten gallons it might be meant for tank that size but it could be too long for your particular tank. So make sure the length works too.
Always read the directions thoroughly!!!! Each heater is different. The directions will explain vital information such as the maximum/minimum water line for the heater. If you accidently completely submerge a heater that is not meant to be completely underwater, you could electrocute the fish.
Here are some tips I have learned about buying heaters:
Watch out for "mini" heaters. You see one and say "that would be great for my tank!". However it may just raise the water temp a few degrees. If the temperature where you live isn't fairly high and fairly constant, these mini heaters will not work for you.
Sometimes cheaper is better. You might think you should get a better quality expensive heater. For a large tank with lots of fish, you might want to do that, but if you just have a small tank with a few fish, go for the cheaper ones. I'll explain why in a moment.
Be careful about reviews and ratings. As some fish hobbyists have said " no fish keeping equipment is really 100% reliable". There isn't a single brand of heater that doesn't have problems. Generally someone gets a heater that works great, but someone else is going to get one that malfunctions, or you might even get one dead right out of the package. So save the receipt just in case you need to return it. So the point of what I just said is don't go looking for perfect reviews and five stars. You probably won't find them.
Now we can get into why I suggested buying a cheaper one. Yes, if you read reviews they aren't always as reliable. Sometimes they are dead right out of the box, and often the temperature doesn't stay 100% consistent or the settings are a little wonky (for example you might set it at 78 and get a water temp of 74 instead). Sometimes they don't last terribly long. However most of these problems are no big deal. If it is dead right out of the box return it and get another one. If the temperature fluctuates a few degrees your fish will still be just fine. If the settings are not 100% correct just fiddle around with the controls for a bit until the water temp is where you want it.
I suggested a cheap one as opposed to a super expensive one because the problems more expensive ones seem to have seem to be a lot worse. True, they malfunction less often and tend to last longer but when they do malfunction it tends to be nasty. I have read reviews saying they exploded, or the heater suddenly got so hot it basically fried the fish. Most of these malfunctions end with dead fish.
I would rather go with a heater that's worse malfunction is it stops working (all heaters will at some point) or the temperature is slightly cooler than I want. If you really are worried about the heater suddenly dying, you can keep a spare ready.
So I would recommend a cheaper heater and probably one of the submersible kinds.
Or if you don't want the hassle of heaters, buy a gold fish. They are cold water fish and do not need heaters. (of course goldfish need large tanks and have all sorts of special needs so even without a heater they can be pretty complicated.) (I don't know who said keeping fish is easy. ) (its not as easy as a lot of people think) (of course once you know what you are doing it isn't ridiculously difficult either)