Submersible heaters are the most commonly used heater in use today. Most are now made from shatterproof materials and often come present at the optimal temperature for tropical fish. Some have temperature controls by degrees, while others merely allow for adjustment up and down from the preset level. While I recommend choosing ones with actual temperature markings on them, if you are on a tight budget, the +/- adjustment types will generally work quite well.
AquaTop makes two product lines, a high end one composed of titanium, and the more economical HT series. The HT series is shatterproof, and has a temperature dial that is graded in half degree increments. Although it lacks some of the features of the higher end models, it gets the job done at a very economical price. Good choice for those on a tight budget, or as a backup heater to keep on the shelf for emergency use.
Built from shatterproof materials, the Aqueon Pro line of heaters are suitable for both saltwater and freshwater aquariums. They are fully submersible and are equipped with an electronic temperature controller that is reliable and accurate. Available in five different sizes, ranging from 50 to 300 watts, they will accommodate the needs of a range of aquarium sizes. Perhaps the best feature of this heater is the customer support. Aqueon stands by their product, offering support for the life of the heater. I've had one replaced myself, no questions asked, after several years of use. Reasonably priced, Aqueon Pro is a solid choice for an aquarium heater.
Made from indestructible titanium, Azoo heaters are particularly good in saltwater aquariums where corrosion is an issue in some heaters. However, they are just as suitable for freshwater aquariums. The temperature setting is graded in Celsius, and includes an automatic shut-off in the event the heater is removed from water while still turned on. I have not seen them for sale locally as often as other brands, which I believe is a combination of the higher price and Celsius scale, which is not as popular in the United States.
Eheim makes solid products, and the Eheim Jager heater is no exception. Constructed of non-breakable materials, they are equipped with a low water shut-off and visual on/off light. Adjustable in half degree increments, this heater is also capable of being re-calibrated by the owner to ensure temperature accuracy. Jager heaters come in 9 sizes, from 25 watts all the way to 300 watts. Considerable debate exists over whether the Jager is full submersible or not. The primary issue seems to be one of government regulations in the United States which require UL approval. This product was originally the Ebo Jager, and when the product line was purchased by Eheim, the UL approval did not automatically transfer. Recent product descriptions are now indicating that it is fully submersible, but I have yet to see the UL information.
The Fluval E series of heaters come with an interesting visual alert system using a colored LED display to indicate temperature ranges. Temperatures can be set to half a degree increments, and the LED color alert will remain green as long as the water temperature remains within the preset range. If the temperature rises, the color will change to red, while a drop in temperature will change the LED to blue. Another feature is an integrated heater guard that ensures active fish can't break the heater. Available in 50, 100, 200 and 300 watt sizes. It comes preset to either 25 C or 77 F, but can be reset to any temperature desired. I've used several of these heaters, and found them to be quite reliable.
The Hydor Theo series of full submersible heaters is shatterproof and has a graduated temperature scale. Like other submersibles, it has an automatic shut-off in case of running dry or removal when still turned on. Mid-priced, it lacks some of the features the higher end heaters have, and some owners have reported issues with replacement when under warranty. Available in nine sizes from 25 to 400 watts.
Marineland offers the Visi-Therm series as an economical option for those wanting a full submersible heater. Made from break-resistance glass, they should be used with a heater guard in tanks with active fish. They come pre-set and are accurate to one degree. What I ike is the click feature that lets you know when you've set moved the temperature by one setting. Marineland is a trusted name in the aquarium industry, and their heaters are no exception.
ViaAqua is another of the economical priced heater product lines. Made from quartz glass, they are break-resistant, but not shatterproof, so consider a heater guard if used in a tank with large active fish. The temperature control knob is plus minus, but it has a degree temperature scale on the front that is easy to read, and allows for accurate temperature adjustments. Not a lot of frills, but a solid performer for anyone who needs a low-cost heater.