I think your right man. Something is off with those heaters. I run an older aqueon 250 in my 6 foot 100 gallon, and two newer 200's in my 75 gallon, and a 40 breeder that is down close to a concrete floor. The room drops into the low 60's easily and dipped into the 50's when the power was out. I also have an old ebo jaeger 250 in a 72 bow front. All of these tanks easily maintain 78-80 degrees. They do take a long time to heat up without the help of a little warm water during water changes but 300 seems big enough for a 75 that it shouldn't struggle that much IMO. I even have a 100 watt in a 40 breeder shrimp tank that runs about 73-74 on average.