There are two basic types of aquarium heater (and a sub type)
Also other heating elements for small tank/bowl use such as Bettas

Another term for the automatic heater is a Non-Preset Heaters, while the Thermostatic heater is also referred to as a Preset Heater.

As for heating requirements, I recommend 25 watts for every 10 degrees of ambient temperature per 10 gallons you need to raise your aquarium temperature. EX: If your home is 68 degrees and you have a 40 gallon aquarium, to reach a temperature of 78 degrees you would need a 100 watt heater. Often when heater problems occur it is due to the heater being of the improper wattage to meet heating requirements of an aquarium in the location it occupies. Too high a wattage can result in rapid temperature rises that can result in cooked fish if you make adjustment mistakes.
Also, if your room is warm (such as in summer months) you may end up with aquarium temperatures higher than your settings (example: a room that is 85 will keep an aquarium at 85 even if your heater is set at 78 F).

It is important of all heaters, regardless of type to maintain good circulation around the heater for the best results for temperature control and accuracy. I have used (& even designed my own) heater modules for in-line heater applications and I will be the first to admit these do not properly dissipate the heat, especially with titanium heaters.
Even though I really like the durability of Titanium Heaters, these types of submersible aquarium heater are best used inside the main display aquarium where the heat will dissipate more rapidly than in a sump or similar enclosed space. I have witnessed Titanium Heaters (of different brands) over heat and burn out when used in small sumps. I recommend using a glass submersible heater in sumps or similar as glass submersible heaters due not seem to be as sensitive to this problem.

Set Up Suggestions
When ANY heater is to be initially used, you should place the heater in the aquarium for 20-30 minutes prior to plugging it in so as to allow the temperatures to equalize internally. With low end Automatic Hang on the Back heaters make sure that the water is up to the water line, if too high or low this can cause heater failure or malfunction.
With pre-set submersible you need to make sure that the heater is submersed at least to the minimum water line.
Since Automatic Heaters generally do not have temperature pre sets, all inputs of temperature control on the dial should be made in increments of not more than ¼ turn, with 1/8 turn being better.
With Pre-sets and digital heater controls, I recommend to not change temperature more than 4-5 degrees F per day if fish are present (this doe not matter if there are no fish or other “creatures” present). Also make note that temperature pre-sets are just a guide and that it is quite common to set a heater at 75 F and have the actual temperature be plus or minus 3 degrees F.

PRESET HEATERS:


The Thermostatic or Preset Heater has a built in thermostat (or a remote sensor as is common in many titanium aquarium heaters). These heaters may use a digital thermostat, usually monitoring the impedance of a thermistor (a resistor used to measure temperature changes in the aquarium, relying on the change in its resistance with changing temperatures), or the potential across a thermocouple (a temperature sensor used to measure electric potential difference).
There are many models of aquarium heaters available today that come preset or with a temperature gauge on the side of the heater or on the control knob to help the aquarist set the temperature. The aquarist should allow the heater to adjust to water temperature for 30 minutes before plugging in this type of heater to allow the thermostat to adjust. Use these settings on the dial as a guide to make fine adjustments, as they should not be depended upon for total accuracy... Though some manufacturers make other claims, these gauges are at best accurate within two or three degrees Fahrenheit (one to one and a half degrees Celsius), and generally get less accurate over the life of the heater. An aquarist should always check their heater for accuracy and not assume that because it is set at 78 F, it will stay at 78 F.

It is not un-common for even the best of these heaters to be off from their “pre-settings” by a few degrees, so for instance if your pre-setting takes a setting of 76 F to maintain 78 F (& 78 is what you desire), you should use the setting of 76 F.

I also recommend turning off all heaters when water is to be changed that would exposed any part of the heater sensors or heating elements (which is usually most water changes). I recommend waiting at about 10 – 15 minutes for water and heater temperatures to equalize before re-starting a heater after a water change

Titanium/ Stainless Steel/ Remote Sensor Heaters

A subtype of the Preset heaters is the Titanium and Remote Sensor Heaters such as the Via Aqua Titanium heater The advantage to these heaters is twofold; one is that they tend to be even more accurate by having the sensor far from the heating element, and two; the Titanium (and also Stainless Steel) heaters are much more durable and difficult to break especially with large fish such as Oscars. These heaters also resist breakage due to aquarist mistakes such as leaving them on during water changes or dropping rocks on them (although Titanium heaters still can be damaged by leaving them on during a water change).

I think Titanium Heaters are your best choice for very large aquariums and for aquariums with large and destructive fish such as Arowanas, Oscars or even Turtle tanks (provided there is adequate water volume).
Titanium Heaters are NOT as good a choice for placing in confined spaces such as sumps, low water tanks (such as Viviariums) or filters such as the ReSun Internal Wet Dry Filter as Titanium heaters tend to easily overheat in confined spaces. In the case of a sump, the reasons for spending extra for a Titanium heater which are impact resistance and the remote sensor are lost in this small space where the remote sensor is of little advantage and no fish are present.

Please note that my experience with the Marineland Stealth Heaters indicates that they should also not be used in confined spaces or low water volume applications. I have also noticed (in part from feedback from others in the professional aquarium maintenance business) that the Marineland Stealth heater is not as accurate or as durable as the higher end Via Aqua Titanium Heater (or similar ReSun) or the very high end Pro-Heat II Titanium IC Heaters.


AUTOMATIC OR NON-PRESET HEATERS (HOB Heaters)


The Non-Preset or Automatic heater such as the basic Hawkeye or Radiant Heater has a very remedial thermostat; or better they have metal contacts that are tightened by turning the dial (metal contacts installed on bi-metal lead, which are brought together with an adjusting screw). When the heater is "turned up" or tightened, it turns on for longer periods of time. These heaters need adjustments between summer and winter. Examples of this heater are the Radiant by Hagen, or even the old Metaframe heaters. Sometimes these heater are all some aquarists can fit or afford on their tank (although Thermostatic heaters have come down in price a lot), but I have seen many an aquarist cook their fish with these, as subtle changes in the dial often over correct and a sudden ambient warm spell is often not corrected for.
With these heaters it is imperative that the water level is kept up to the proper level as marked on the heater or they may crack, it is also important to check these for seasonal variations (Summer/Winter), as these heater adjust poorly for these. These heaters should not be used in a room or garage with wide temperature swings as they will not generally adjust properly. Adjustments with these heaters should be made at no more than 1/8 turn at a time then wait for about two hours before the next adjustment.

As with other heaters it is a good idea to place the heater in the tank and wait for 30 minutes before plugging in (although this is less important with these automatic heaters).
Adjustments should be made very gradually and it is easier to adjust these heaters if you start out at our near the desired temperature, otherwise you may be in for a rollercoaster of adjustments.

Other Aquatic Heating Devices

As for heating betta in a small tank or bowl, this can be difficult with even the smallest aquarium eaters as they tend to be less accurate in very small volumes of water.
I have used (also for my clients) infrared reptile lamps or infrared heating elements (these are generally more pricy and not worth the extra money for a fish, unlike a reptile) placed in desk lamp s over the tanks with the distance set according to the temperature desired. The distance will need to vary based on ambient room temperature. The nice part about using these reptile infrared lights is that they do not interfere with the day/night cycle of the fish the way a white light or even a blue light will.
These are actually quite accurate when used correctly and more importantly they do not upset the day/night cycle needed by fish.



SUMMARY:


When choosing the right heater you need to factor your budget, tank size, fish size potential, ambient room temperature variation, and temperature sensitivity.
For example: a 10 gallon (35 liter) aquarium with platties and guppies would probably be fine with an inexpensive Automatic Heater, the same for a small Betta Tank. But if this tank was in say a shed where temperatures vary greatly, even this example would be better of with a Preset heater (for Bettas I have also used infrared reptile lamps in desk lamps to maintain temperature successfully while still allowing day/night cycles).
Another example would be fish such as Oscars; with Oscars I would recommend the stainless steel or Titanium heaters as these fish can get rambunctious.
A third example would be marine fish; marine fish generally are not accustomed to much if any temperature swings so a Preset (thermostatic) heater would also be in line here.
Finally for any larger aquarium (40 gallon +), a Preset heater just makes more sense in my opinion.


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