[D-raw]

What's a heater that is strong enough to bring a 300 gallon tank up to 80 degrees?

 

[ngrubich]

It will probably be easier to have two or more smaller heaters on opposite sides of the tank to heat each end, and have flow going over each of them to distribute the heat more efficiently.
In my 90 gallon, I found it to be more convenient/easier on the heater to have a heater in my overflow box and another heater in the return section of my sump. The way I have mine set up, my tank stays at ~75-76 degrees throughout the entire tank, and the heaters aren't constantly on.

 

[D-raw]

Yeah I think I'll go with a couple of 500w heaters from ebay

 

[jan db]

I have 2 jaeger 300 watts on my 340 and its plenty.

 

[Diana]

Depends also on the room temperature. If the room is kept cool, then perhaps as many as 3 heaters, equally spread out and in areas with good water movement.
I would put a thermometer at each end, too. Make sure they read the same before you put them in the tank.