Here is a nice little trick I have used when picking out heaters...
USE THIS TABLE and follow directions below to find out the right heater size for you.
Gallons/Liters 5ºC/9ºF 10ºC/18ºF 15ºC/27ºF
5 gal/25 L 25 watt 50 watt 75 watt
10 gal/50 L 50 watt 75 watt 75 watt
20 gal/75 L 50 watt 75 watt 150 watt
25 gal/100 L 75 watt 100 watt 200 watt
40 gal/150 L 100 watt 150 watt 300 watt
50 gal/200 L 150 watt 200 watt two 200 watt
65 gal/250 L 200 watt 250 watt two 250 watt
75 gal/300 L 250 watt 300 watt two 300 watt
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 column 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 desired 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 F
Desired Water Temp = 77 degrees F
--------------------------------------------------
Heating required = 9 degrees F
Tank Size = 20 gallon
Now look up to the chart to find that a 20 gallon tank with 9 degrees Fahrenheit of heating required will need a 50 watt heater.
You can buy a nice heater at any local fish or pet store. They usually have a wide selection from 25 to 300 watt heaters for aquariums. You could also check Ebay, or google Dr. Foster Smith.
HOPE THIS HELPED!
P.S. The chart did not come out the exact way I expected. It is slightly slanted. If you have a problem reading it, just tell me, and i'll try to fix it.
