if you have fish that require 80+ degr temps, then you probably want to keep a heater. most other fish can handle not having a heater, and it is perfectly natural to have water temp that goes up and down a few degrees. sudden temp changes are bad, a few degrees over the course of the day are not to be worried about. you will find even without a heater a tank will generally be a few degrees over room temp as the air pump, filter, lights, etc put heat in the water. in my 25+ years of fish keeping you can probably add up all the time ive had a heater in a tank, and be at less than a year.