To make temperature reading easy, consider purchasing an adhesive temperature strip that can be applied to the outside of your aquarium glass. They are inexpensive and available at your local fish retailer.
Monitor the sun – Too much sunlight can cause algae growth. If you are installing a new aquarium, place it away from a window. If your existing aquarium is near a window, use a shade to reduce the amount of ambient light.
Avoid heat — Never place an aquarium above or near a heat source or air conditioner. You want to keep the area’s temperature as stable as possible.
For all species of fish, there is a middle ground in which they are happiest, and they must be kept within that range for optimum health. Examples: Tropical fish – 72° - 80° F (optimal is 78° F), Common Goldfish – 65° - 68° F, Fancy Tail Goldfish – 65° - 72° F
Submersible heaters are the most popular. Some are adjustable and others are pre-set at 78° F for your convenience.
Be sure to purchase a heater that is properly rated wattage-wise for your tank.
Never remove a heater from the water if it is still turned on. Unplug it first, let it cool and then remove it.