You need 3 to 5 watts per gallon. If you have a fairly well sealed acrylic tank, 3 watts per gallon should work fine. If you have large slot openings for HOB filters or an open sump on a wet/dry, look towards the 5 watt per gallon sized heaters. On large tanks, I also recommend using 2 smaller heaters that add up to the wattage (or over) that you need. This will give added protection to the tank if a heater dies. Also, if a thermostat locks on on a smaller heater, it can be caught before it turns your fish into soup.