Multiple small heaters adds some safety factor. There are plenty of stories of heaters that went nuts and heated tanks up into the 90s. And also plenty of stories of heaters just quitting and tanks getting very cold. If you have several small heaters and one overheats, it wont cook your fish. If one quits you wont wake up in the morning to fishsickles.
I've been running 2 300w in my 150g for a few months. I noticed when I just had the 1, the heater was always on so I decided to get the 2nd one. Also 1 heater malfunctions I have the 2nd one.
I use 1 inline 300w on my 180 and it keeps it 80 even when the ambient temp is 60-65. It doesnt run as much as I thought it would either. With 3 canisters I think I am getting enough current to keep the temp evenly dispersed.
