No experience with Fluval heaters, but a good formula for determining watts is:
[(desired temp - room temp)/10] * gallons * 3 to 5 watts (varies heater to heater) = total watts required
Ex: [(82F desired temp - 74F room temp)/10] = 0.8; 0.8 * 120 gallons * 3 to 5 watts = 288 to 480 total watts required
All heaters are "cheap," so it's wise to invest in a quality controller, use multiple smaller heaters or both to prevent/mitigate failures.