I have a 50 gallon tank. My temperature has been pretty stable at around 77.3 . Over the past week the temperature has varied from 76.5 in the morning, after the lights were out during the night, to 78.1 in the evening. How much variation in temperature is acceptable?
Temperature fluctuation is normal as long as it remains incremental. I think your temp. range is fine, but for arguments sake I would go with 2 smaller heater over a single larger wattage. Two 75w should suffice. Temperature range from 76F-84F is acceptable, though try and target 78F-80F just to allow leeway.