i would imagine it depends on how much flow the fuge has... it may get too hot... :notsure: although i'm not positive of that...
unless its a temperature sensing heater that shuts down when it reaches the set temp.
you could get two heaters, put one in the fuge and one in the sump... this way the heating is split by the two and they dont have to work as hard to maintain the temp. it's also a great system for back up, incase one dies...