It's just a matter of how much the temperature swings. A small heater like that will keep it at 78 as long as the temperature outside the tank doesnt get too low or way too high. My preset heaters work fine even with the air vent blowing on them in the kitchen.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.