75 gallon tank, should I use 1 or 2 heaters?
I have two 300w heaters in my 84g. They are very efficient and don't run as often as the smaller heaters used to.
Also with that temp spread and only obe heater you'll lose heat fast and regain it slowly. Two would be much better.
I use the digital Fluval heaters. Have been rock solid for years, include a temp display and color-coding so you can see what's going on at a glance.