I've been running a 150w heater on my 29g sw
and have had no issues. You also have to account for the length of the heater. If you get a larger wattage they are most likely gonna be really long and much more noticeable.
I run 50w heaters in all 3 of my 10g tanks, again with no issues. I have a 250w in my 55g sw
, NONE in my 29g biocube and 2 X 300w in my FW