Heaters

There's no real savings in a cheap heater that's not completely submersible. There are many better ways to save money when you're setting up an aquarium than by buying a "bargain" heater. Adjustability is another issue with aquarium heaters. Make sure you can adjust your submersible heater easily with one hand. If you have to remove it to adjust it, you'll come to curse the thing.
Right-tighty, lefty-loosey. A clockwise righty turn tightens the setting i.e. reduces the gap i.e. lowers the set temperature. A counter-clockwise turn opens the setting i.e. widens the gap i.e. raises the set temperature. Easy for you, but forever counter-intuitive for me. I still have to freeze for a moment and close my eyes, visualizing the action. You see, it's just the opposite of "right: raise. left: lower" Oop! have I spoiled it for you too now?
Unless you just can't manage it, the best place for a heater is horizontally suction-cupped to the back wall, just above the gravel, with the cord travelling inconspicuously up a rear corner. If you leave the heater tube lying in contact with gravel there's a chance it will overheat. And if you station it vertically, you are bound to burn out the heater, sooner or later, by neglecting to unplug it before you begin a partial water change. Not the first ten times, but sooner or later it's inevitable, unless you're a great deal more methodical and unhurried than I am. Look into purchasing an open mesh sleeve that fits over the heater tube, if you have secretive hiders like Lories and Loaches that are prone to squirm in under the heater and burn themselves.
 
Working heaters heat the air inside the tube, switch off, then switch back on. Since the air heats up and cools down far more rapidly than the water outside, it takes many cycles on and off to raise the temperature of the surrounding water.
 
Mike Wickham pointed out in his great beginner's book, The Complete Idiot's Guide to Freshwater Aquariums, that the better quality heaters have small magnets on the bi-metallic strip and on the stationary contact. As temperature drops and the contact points get closer, magnetic attraction comes into play and snaps them together instantly. When the heater turns off and they separate, they pop apart. No more arcing as the circuit is closing. Arcing builds up carbon deposits on the contacts and ages the heater, until eventually the contacts misfunction. This was a good tip for techless me. Of course, even better nowadays is the electronic heater that merely kicks on, raises the water temperature, then kicks off. The high price of an electronic heater is a consideration, though.
 
How many watts? When you're matching the wattage of the heater to the volume of water, don't go for "overkill." When the heater malfunctions, you want it to take some time before it heats the water to lethal temperatures. The longer it takes to overheat, the more time you have, on your routine checks, to realize something's amiss. In ordinary use, a heater can maintain a temperature using fewer watts of energy than it would take to raise the temperature of a volume of water. Judging how many watts you need for a certain volume of water and substrate depends on how many degrees above ambient room temperature you need to raise the tank water. An old-fashioned rule of thumb was, two watts to raise each gallon five degrees above the ambient room temperature. So a ten-gallon tank kept at 75^oF in a room that was 70^o would need a 20-watt heater. Less wattage than you thought, maybe? Larger tanks, 55 gallons and up, are most safely heated by two heaters, each rated at half the necessary wattage. If one of them sticks in the "on" position, the other stays off, giving you that extra time to act.
 
With all the things that can go awry in fishkeeping, a malfunctioning heater that boils your fish is the one avoidable disaster you'd most kick yourself for. I take the heaters out of all my tanks once summer's warm weather has securely settled in, and before I replace them in the fall, I check them over.
 
 
Other sources of heat... and cooling. In practice, your lighting will also contribute to the heating, though new compact fluorescents deliver more light and less heat for the watts, and LED lighting is even more efficient.
In summer, when tanks can overheat, more heat will be lost through evaporation and convective air currents from a tank that's open-topped than from a tightly enclosed system. Evaporation is cooling: drafts in the room have little effect on heat loss from the dry glass walls of an aquarium, but in a heat wave emergency you can wrap the tank in wet sheets or towels, and evaporation will help cool the system. A fan blowing on the wet towels will speed the evaporative cooling.
 
Link. This review of aquarium heaters describes the range of heater types.