About the Automatic Aquarium HeaterMake controlling the temperature of your aquarium easier than ever before with the Automatic Aquarium Heater. A temperature control dial automatic safety shut-off switch and shock-resistant glass make this heater ea sy and safe to use.How to determine which size heater you need:Subtract the average temperature of the room the aquarium is located in from the temperature you wish to maintain the aquarium water at. Find the size of your aquarium in the left hand co lumn and move to the column that shows the number of degrees the aquarium needs to be heated. If the heating requirement is between levels move up to the next larger size.In larger tanks or where the room temperature is significantly below the desire d water temperature two heaters may be required. Heaters should be installed at opposite ends of the aquarium to heat it more evenly. Example:Average Room Temp = 68 degrees FDesired Water Temp = 77 degrees F----------------------------------------- Heating required = 9 degrees FTank Size = 20 gallonHeater size needed = 50 watt