I didn't know about those...
If you can't adjust the heater honestly I would get a different one... espescially if the tank is only housing fancy goldfish. You will thank yourself for it if you need to turn the temp up or down for disease treatment.
Plus if the D.O. is insufficient at 78 and you wind up having to buy an extra air pump... you might as well have bought a new heater!!!
Also bear in mind that heaters are really 'upsold'... pet stores reommend 5w per gallon when what you actually need is half that. It will heat just as well and if the thermostat ever breaks it won't even have the wattage to cook your tank. I'd recommend 150W for every 50g.