Here is the calculation for determining the size of the heater you need for your tank.
For Tanks up to 30 gallon its 5 Watts per gallon. For Example a 20 gallon tank would be 100 watt heater-For Example: 20 gallon x 5 watt = 100 watt heater.
Tanks over 30 gallons are calculated at 3 watts per gallon- (You may need to round up wattage to find correct size heater.) Example: 40 gallon x 3 watt = 120 watt. But 120 watt heaters either wont exist or be rare to find so you go up to 150 watt for the tank since those are easy to find.
Another thing to keep in mind is that with large tanks usually 75 and above you will most likely have to split the wattage between two heaters to get the adequate heating. So for example a 100 gallon tank x 3 watt = 300 watts. But You should get either a 200 watt heater and a 100-150 watt heater.
If the area you have the tank in is cold like in my case in my fish cave(basement) it can get cold so no matter what I use the 5 watt per gallon rule for even the large tanks.
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