not sure about the specs of the tank but as far as heaters go generally.. i dont even have one lol, well i have a submersible one but i dont use it i just left it in my tank because it has algae on it and my clown godyl oves to sit on top of it but i actually dont even use it. my tank doesnt drop below 74.. as far as the water level being too high with the heater the line is a recomendation for proper function, like the lin in a laundry detergent cup fill to here give or take a little bit. if its that big of a difference with the heater you might want to try a submersible, but if its too close for comfort then you already know what to do lol.. id take it out..do you have a thermometer? check your temp without the heater if its stays between 72-79 you probably dont need it, your lights will add heat to the tank thats what mine does.. its about 74-75 when my lighs are off then goes up to about 75-76 on its own with the lights on then back down when they're off. kind of like the natural ocean. your not going to get it at exactly "77.5" all the time, its going to fluxuate, hot summer days or cold winter nights etc
I;m so deep that if water tried to listen then the rivers would sink...knowledge is power, get ya weight up!