submersible

Would it be better to buy 2 smaller heaters for a 55 gallon tank for each end of the tank or would 1 300watt for up to 75 gallons heat the tank evenly?? Thanks!!!

My 15 gallon seems to be hold up at 80 degrees. I don't know what brand the heater is but it is a submersible. Seems to work great but I wonder if 80 is too much for a tropical tank?   Then my 10 gallon has a tetra whisper heater 5-15. The tank has been sticking around 77.9 - 78. Would that be...