🚀 go-pugleaf

Yea,

That's a good question. I have a 40gal[M]. with a 100watt, that's 2.5
and it keeps the temperature pretty stable...

but oddly enough I have a 100watt in my 26gal. reef. It seems to 
a harder time keeping the temperature at a steady level.

So I think the 2.5 is really a minimum rating depending on other variables.
i.e. house temperature where it's located in the house.. My reef is on a tile
floor right near the front door. So it gets a lot colder at night in that
hallway vs. my Main tank which stays at a fairly level 79.

Dave.