The Eheim 250w heaters are good for up to 159 gallons.
A 300w Eheim will definitely heat a 180 just fine by its self.
I am certain a 250w heater BY IT SELF would have no problems at all heating a 180g tank.
You need to do some reading and some trial and error...on being a cheapo.
Ive got a single 250w heater in a 125g tank right now. Tank is in a basement that is a constant 70-71 degrees.
Tank water is a constant 80.3 degrees give or take .4 degrees. Ive seen the heater kick on 3 times in 10-12 minutes for
30-55 second intervals.
Yes, another Eheim owner speaks up because he has firsthand experience with the item! Since I have personally used the item I too can say I feel that a single 300 watt Eheim would work fine in a 180g.