Most marine systems house tropical warm-water fish since they are the most colorful. These same fish also happen to be quite temperature-tolerant, since their native home is usually a shallow coastal area where water temperatures can fluctuate several degrees, even within the same day. So why bother with a heater? If the temperatures in your home are good enough for you, they should be good enough for the fish, right? Wrong.
Even the most temperature-tolerant creatures need some consistency. Wild fluctuations in water temperatures in a closed system stress the fish; not only do their nutritional requirements fluctuate with the temperature, but temperature variations can cause drastic water-quality fluctuations or parasitic outbreaks that your pets cannot tolerate. The temperature will have an effect on the oxygen availability in your tank. So, the question is not whether to get a heating system, but rather how to buy the right one for your aquarium. They are not that expensive, but they are worth the investment to protect your pets.
Heaters come in a few varieties, but for most home aquaria submersible heaters are considered the best and most reliable kind. Always make sure when you buy your equipment that it is for use in a marine system, too. Most submersibles are made of glass and should be placed near the bottom of the tank. They heat the water, causing a rising stream of heated water that helps circulate water through the tank. Submersible heaters should be kept away from the sides of the tank and from sand, so you may have to hang it with suction cups. If this is the case, remember to clean the suction cups as regularly as you do the filters.
You'll have to know the water temperature so you'll need a thermometer as well. Those that attach to the outside are influenced by the air temperature and aren't as accurate. Units hung inside the tank are better overall, but should not have any metal parts and should not be of the mercury type. The most common and preferred are alcohol thermometers. If your tank is larger than 60 gallons, you may want to consider getting two and hanging them at either side of the tank, away from water flowing up from your submersible heaters.
Getting a heater is a little more difficult than it first may seem. If you don't want to boil your fish, make sure your heater doesn't stick in the "on" position, or that it is so powerful that it is constantly switching on and off. You can usually check the performance of your heater by watching the pilot light. Although you should be sure about wattage and other such specifications that your own system requires, here are some general guidelines to keep in mind:
Guidelines for Heating
Finding the right heating system for you may take a little more time than you thought, but after that, it's an essentially worry-free and hassle-free part of your aquarium setup.